Great Idea!!

Inner Socket fit outer socket reflector cavity and vice versa; metal portions of both sockets match each other right down to the extra wide notch that appears to align/orient the outer socket bulb. (The inner light housing does not have the protruding tab for orienting a bulb)

So the next step is to get the bulbs and try the fit for internal clearances......I'll post results of that test.

I lost you here:
"get another outer-type socket and tap its extra wire into the outer socket's bright-mode (brake/turn) wire"

If the goal is to get the inner taillight brightness matching the outer taillight brightness (only) would the extra wire be best just capped off?

Said another way ......turning the inner light into a brake/blinker would add additional load to the light circuit?

I like your thinking changing the light socket to get even lighting and the benefits of reducing load on the circuits as well as protecting what's left of my 65 Dart sad lenses from further deterioration due to bulb radiated heat.

_________________
1965 Dart 110k, 225, Carter BBD Super Six, 2 1/4 single exh., sbp manual scarebird front disc, 7 1/4 rear 2.94 sure grip, 14 x 4.5 OEM wheels, 833 OD with hyd. throwout bearing, HEI, electric fan, ram air/heated air, Accusump. http://plymouthcarclub.com/

The other thing to check is the focal length—that's important. What you'd want to do is get an 1156 bulb, install it in the inner socket, and hold the inner and outer sockets right next to each other to see if the outer socket holds its 1157 bulb with the bright filament at the same height as the filament of the 1156 in the inner socket. The 1157 has two filaments; the one closer to the glass is the dim filament and the one below it, closer to the base, is the bright filament. You have a little more leeway with LEDs than with filament bulbs, but it should at least be pretty close. And you may have to experiment with different clock positions of the socket in the inner lamp to make it fit and light up the reflector/lens correctly. Once you find a position that works, make a line across the socket and reflector body so you can easily align it in future.
You could do that if you want, but why not double the size of your brake light?
Two LED bulbs take a fraction of the current of one incandescent bulb; you stil net out at much lower load on the circuit.

Forgot to mention: You will need to swap in a new turn signal flasher, that's this one (2-prong like original; connect its ground wire conveniently).
That, too. Goes especially for '64-'66 Valiant-Barracuda cars; the angle of their lens means the part directly above the incandescent bulb tends to melt/divot.

In case I haven't previously mentioned it: you have to be really picky with LED bulbs. Most of them on the market are unsafe junk, including all of them you can buy from the likes of SooperBrightLeds, eBay, and places like that. They all claim they'll give you the world on a gold platter—most of them will make you more(!) likely to get hit in traffic. Please see my detailed discussion of the matter here.

There are a couple of reasons why I listed two red 1157s to try: first is the physical fit issue; one may fit where the other doesn't. But even if both fit, they're still a basically different kind of light source than the lamp was intended for, so just because it physically fits doesn't necessarily mean it will work safely and effectively. You'll need to assess the performance in your particular lamps by comparing them side-by-side with the original incandescents as reasonably well described here.

The bulb for back-up lights is this one (haven't yet found one it doesn't work well in, though in '63-'64 Darts you have to make sure it's rotated with one of its faces pointing directly at the lens).

Things get trickier with the front turn signals. You have to look carefully at the lens. If yours is the kind of lens that has a circular bullseye in the middle with concentric rings around the bullseye, that's a Fresnel lens and it magnifies and distributes light by "looking" directly at the bulb, so you need a bulb that emits light straight out the front. There is no legitimate, effective 1157 bulb that does this in any colour other than red, and a lot of our cars have these Fresnel type front lamps, so for them there is no effective LED bulb upgrade. Note that a variant of Fresnel optics uses straight lines instead of round rings; many back-up lights have these including '60-'63 Valiants, '63-'65 Darts, and others.

If your front lenses don't have the bullseye-and-rings optics, and the bulb has a reflector bowl behind it, then you can try out these (per the test/comparison method linked above) and see how they do.

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

I'll throw this in here........

You've probably have all heard that LED's light up much faster than incandescent bulbs.

Supposedly useful for turn signals and brake lights.


And then just said 'Well, so'.........


I recently replaced one of two bulbs for the license plate of a ford focus with a LED bulb. (both are now replaced, 2nd incandescent lasted 2 months longer than first (after 15 years of life))

When arming/disarming the alarm, all running lights flash.

The LED light was fully on, then off before the incandescent fully lit up.


So the speed difference is actually noticeable.

_________________
Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

Fit-up in a 65 Dart with inner and outer taillights (outer lights are the brake lights also):
The Philips LED were 1/4 inch less height than the Sylvania however the Phillips OD diameter (at the point where the light socket snaps to taillight housing occur) interferes with the stock light socket snaps.

The Sylvania fit the socket perfectly and mounts in the housing without contacting the lens of the 65 Dart on the outer rear lights (with brake and taillight functions).

I ground a portion of a NAPA replacement socket LS6469 to clear the Phillips and managed to get the assembly to snap into the inner rear taillight housing.

Based on the above fit-up findings I would recommend the Sylvania over the Phillips unless you want to replace sockets and grind a portion of the socket to clear the Phillips large OD.

Light Intensity:
Stock 1056 bulbs were put in the inner taillight to compare with the outer taillight housings with the Sylvania LED installed. On both the right & left sides of the car rear the taillights were somewhat brighter for the 1056 inside taillights than the outer taillights with the Sylvania LED.

When brake lights were applied the outside lights were brighter than the inside taillight 1056 however not enough difference to alert a driver in a following car to brakes being applied in my opinion.

Also tested the Phillips against the Sylvania by installing the Phillips on the inside taillights and comparing the taillight functions against each other. The Phillips inner taillight seemed similar in intensity to the Sylvania outer taillight. As such the taillight functions of each LED were less intensity then the 1056 bulbs.

Before anyone draws conclusions from this test be aware the 65 Dart outer lenses have suffered from the heat of the bulbs thru the years and are likely to effect the results. Also it would be better if some kind of "intensity meter" was used than my impressions.

I'm likely to run LED on both the inner and outer taillight for the 65 Dart and will try changing the outer lenses to draw my own final conclusion.

I'll come back with more intensity results after I change outer lenses if I can find some decent lenses.

_________________
1965 Dart 110k, 225, Carter BBD Super Six, 2 1/4 single exh., sbp manual scarebird front disc, 7 1/4 rear 2.94 sure grip, 14 x 4.5 OEM wheels, 833 OD with hyd. throwout bearing, HEI, electric fan, ram air/heated air, Accusump. http://plymouthcarclub.com/