I am installing a 3rd brake light on my 73 Swinger in the back window. It has three wires, black ground, yellow to right side brake/signal wire, green to the left side brake/signal wire.

I grounded it, and before I hacked into the wires I just pushed small spades into the back of a wiring connection for the yellow and green wires for testing purposes. The wiring harness has the dark green wires for the brake lights coming out both sides, one goes to the right, one goes to the left. The violet wires in the wiring harness are for the backup lamps (according to the FSM).

When I tried the 3rd brake light by pressing on the brake, it worked fine, along with the regular brake lights. The right signal works, but when I activated the left signal, the 3rd brake light blinks along with the left signal. I tried switching the green and yellow wires, same thing (left signal/3rd light blinking). I tried hooking both to one brake light wire, and the 3rd light doesn't work.

Any ideas what is going on?

By the way, I don't like adding this wart to my car, but it is a safety issue with all the soccer moms in thier Chevy Suburbans yakking on thier cell phone oblivious to actually having to pay attention to what is happening on the road. If some big SUV rear ends my car, I'm sure their insurance compnany will total it for some nominal fee without regard to my cost and effort in getting it to its current condition or a long drawn out battle to get more.

You need to run a new wire from the brake lamp switch directly back to the back of the vehicle. The existing wires cannot be used to drive the center brake light, because they are combination brake/turn lamps. Also, if the center brake light you're putting in is a bulb-type unit (not LED), you will throw an overload on the existing brake lamp wiring, which will slow down the speed with which all three brake lights illuminate, which will wind up reducing your safety, not improving it.

There was a glut of poor-quality aftermarket center brake lights for retrofitment on the market in the '80s through early '90s. Most of these were complete junk -- not bright enough, wrong angle of light, and some of them came with so-called "logic boxes" that claimed to let you hook up to the existing combination brake/turn lamps. These logic boxes all became illogical in short order, and almost every such retrofit I've ever seen on the road flashes with one or both turn signals or exhibits otherwise defective behavior.
If you use an ugly center brake light, the result will be ugly. If you use a nice one...it won't be ugly! When picking a center brake light, it's important to make sure you get one that will work correctly with the angle of your backglass, without reflecting off the inner surface of the glass to cause distracting red glare in the mirror at night.

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Too many people who were born on third base actually believe they've hit a triple.

The unit I bought was this one

Supposedly made in IN, but who really knows. It is a bulb type, not LED. I removed the "hi-mount" bracket and bought a 3/8" piece of PVC to use as the mounting bracket, and it fits the back window angle perfectly at the bottom of the back window.

However, since the pupose of this is to save me from the soccer moms and meth addicts driving around with their heads up their *#&, maybe I should bite the $$bullet and purchace one of the ones you recommend?

One question though, when you say "run a new wire from the brake lamp switch directly back to the back of the vehicle", where exactly do you mean when you say the "back of the vehicle"? I assume you mean just run a wire from the brake lamp switch to the unit itself, but I want to verify.

BTW, I appreciate your response and value your opinion.

Arrrgh...yeah, that's exactly what I had in mind when I was talking about the crapola that flooded the market in the '80s-'90s. "Logic box" and all!

The one on my site (you'd want to measure your backglass angle at the location where you'll be mounting it -- a regular plastic school-type protractor will work fine) is considerably more costly, but also considerably more effective (brighter and much faster to turn on when you step on the brake), draws 2w instead of 27w, mounts more easily, is a lot less ugly...if you can put up with the $*()@ who sells it, I do recommend it!

Yeah, I meant run a new wire from the brake lamp switch directly to the new center brake light.

If you can tolerate it, mount the lamp up at the top center of the backglass, where it'll do more good than if it's at the bottom.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Thanks! As much as I hate to spend money, investments in future "collision" insurance can be easily justified.

Dan,
I like the looks of the Hella and I like the idea that it self adheres to the window. I am thinking of actually getting one for my Valiant when I finally get it on the road. My question to you is, what exactly does the mounting kit for this one consist of? Do you need to install a fuse with it?
Thanks in advance for the advice!

Russ

The installation kit consists of long wires, connectors, and the preinstalled double-sided foam mounting tape (you clean the inside of the backglass, peel off the backing paper from the foam tape, press the lamp into place...and you're done mounting it). You don't need to add a fuse; the brake lamp circuit is already protected and this LED center brake lamp adds only 2w to the total system draw.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Do these systems use the same brake light switch, or add a second switch? Seems like most (all?) new cars use at least two brake switches, for redundancy. And given how many cars I've seen with only the 3rd brake light working, it would seem worth the hassle.

The one and only factory brake lamp switch is used. Most cars only have one brake lamp switch, though some GM cars have two sets of contacts (one for the center brake lamp, the other for the combination brake/turn signal lamps).

Adding a second switch would not necessarily improve reliability...remember, every component you add means at least one more potential failure point!

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

True, true--but that's a risk I wouldn't mind--I mean, failure means 1 or two brake lights out, leaving 1 or 2 still working. This decrease in reliability is still a boost in safety.

I disagree. Brake lamp switch failure is not at all common in Mopars. It happens once in a blue moon, and you can detect it by checking that your brake lamps function every now and then. The only practical way to add a 2nd brake lamp switch would be to plumb a hydraulically-operated switch into the brake hydraulic system, as Chrysler did prior to 1960. These switches are very much more problematic than the mechanical switches, and safety/reliability would not be improved.

I think you are worrying overmuch about the extremely unlikely. If you're really worried about brake lamp switch failure, go ahead and replace your switch now even if it's working correctly—use the extra heavy duty switch if you like, for an added margin—and turn your attention to more significant issues.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Me, I'm not worried, I have no plans to do this. I've just seen plenty of cars where the regular brakes don't light, while the 3rd one still works--only thing I can think of is that they are seperate. Most of the time it's Fords I see this way... Could be both bulbs are burnt, not sure.

Don't know how the switches are set up, I was thinking just two in parallel. Classic redundant setup, different power feeds/different wires to lights. Not too easily done at home (without fabrication that is), easy enough for an OEM to design that way though. I figured they would have done it that way, guess I was wrong. But if I were retrofitting a 30 year old car with 30 year old wiring, that's the route I'd take.

All I know is that VW's had significant problems with brake light switches a couple of years ago, I guess they got that problem finally worked out. But for some reason I recall hearing that there was two circuits in there, one to the ECU and other to the lights, and that's one way to trip up the VW: apply brakes, and after a second or two the ECU will idle the motor (regardless of throttle setting--it'll think something is broken and "protect" you). Yet the switch can be adjust so that the brake lights are on always and cruise won't work, yet it'll drive ok, or some combination thereof--go figure.

Anyhow, not trying to draw out the thread...

When I put my 3rd brake light into my '75 Coronet, I soldered a wire inside the turn signal switch - then ran a single wire to the light. That way, my light would go with the brakes or the hazard flasher, I didn't need to chop up the rear harness or use the logic module.

A logic module can be made from 2 SCR's cross-connected.

Why go to the trouble to pull the turn signal switch apart and solder wires? You can just run a wire directly from the turn signal switch to the 3rd brake light with almost no disassembly and no soldering.
Failure points, lots of failure points.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Thanks for the reply to my question Dan. I definately think that I will be getting one of these Hellas once my Valiant is road worthy.

Russ