I have done alot of automotive electrical work over the years. My least favorite aspect is finding broken wires within a loom.
I don't mind so much rewiring a trailer from scratch, or adding accessories, etc. I hate scotcklocks, open crimp "butt" connectors, etc especially under the hood, under a vehicle, etc exposed to moisture and such.
But in the case of tracking down issue like "my left taillight don't work" often times the issue ends up being a broken wire right next to a soldered joint. When I join wires, I solder with a cover of heat shrink wrap. Even if I crimp them with a bare butt connector and then solder they love to break immediately next to the joint. I don't get it. Why is this happening?
I built a trailer for my garden tractor some years ago and have had several such failures.
At work, I work on 3 ton international dump trucks, (plow trucks) you guys wouldn't believe from all who have had their hands on them how many such splices there are on these trucks, many 6" from the next.
But the joints don't have to be soldered for this to happen. Even just basically crimped joints do the same.

This is one of the reasons why soldering is not recommended for applications where the wires are going to be vibrating constantly, like in a car. See here and here and here.

Scotchlocks are of the devil. These are a whole hell of a lot better, as reviewed here.

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

The accepted basic standard for aviation is using only double-crimp terminals. The aviation terminal is designed for vibration and to reduce pull directly on the wire strands. The terminal inside is "stepped", with the in-most smallest diameter to single-crimp the bare wire strands, and the outer metal-reinforced section of a greater diameter to crimp tightly on the insulation, in order to mechanically take stress off the current carrying wire. Aviation terminals may also be purchased with hermetically sealable casings. I think the marine/boating industry uses a variation of their own, anyone?? Is anyone aware of what NASCAR uses? The plastic automotive crimp terminals may look like they crimp on the insulation, but that is just wishful thinking.
In aviation we try to never solder a terminal or splice, because of the brittleness/breakage problems.
BTW: vise grips, pliers, and diagonal-cutting pliers are NOT crimping tools.
The FAA handbook for generic airplane stuff like this is the FAA "Advisory Circular" publication "AC43.13-1B". It is about 200 pages (I recall), and a FREE download somewhere at FAA.Gov, (your taxpayer dollars at work), or available on paper for a price.

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"Louise", a 1976 Dart Custom project, (now sadly reverted to being just an "organ donor" to our other project Darts.)

I have to rewire my trailer as soon as I can get it out of the backyard without fear of getting anything stuck getting to it, and get my ariens and cub cadet off of it. I built it from scratch around 2010-2011. All the led lights still seem fine though.
Also gotta go thru one of my 80s Dodge truck wiring harnesses that I have around here. The original on the truck is a hacked train wreck, as is the one my son has from a ramcharger, and the one that I have from an 84 "heavy emissions" D350 out of the local yard to make 1 dependable harness for my 85 /6 D150. The original and to a lesser extent the ramcharger harness are a previous owner hack job, the 84 harness mostly just has weather cracked insulation right near some of the connectors, though it does also have 2 fusible links that need fixed. Definitely the best of what I have. There was another 84 d250 in the local yard whose harness was slightly worse than the one that I got. Any I find will probably be as bad or worse than any I already have.
I ordered bags of various packard 56 spade terminals online, just gotta wait for my packs of various uninsulated butt connectors to get here so I can get started on that.

If your crimped and soldered butt connectors are breaking right at the butt connectors there are two possibilities
1) you are applying too much heat - solder and the solder is wicking inside the insulation in a length beyond the butt connector. You can confirm whether not not this is the issue by stripping away the wire insulation beyond the butt connector and determine if solder is present.
2) you are applying too much pressure when crimping the terminal and fracturing the wire strands. If the break is just inside the connector and matches to the outside edge of the crimp, this maybe the cause.
Crimping is used in the aviation, automotive and shipbuilding industries, but in production environments automated crimping machines with calibrated controls are used. For home manual crimping: get a good crimping tool, not one of those multi purpose wire stripping - crimping tolls, a dedicated hand powdered crimp tool, ‘crimp then tug’ meaning crimp to compress the connector then check with a good tug on the wire. Practice with a empty connector, it may not take as much pressure as you think to compress the connector.

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Doo Ron Ron and the Duke of Earl are friends of mine.....
https://www.youtube.com/watch?v=uX8Nj8ABEI8

After I got done rewiring my Valiant I bought one of the ratcheting crimpers. What a difference it makes. I have 5 or 6 different pairs of crimpers, and the ratchet is definitely the best. It has dies in it that compress the whole connector, not just crimp a small area. IIRC, it was around $30 on Amazon.

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Official Cookie and Mater Tormentor.

Such as this one.

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