It has to be a ground problem.
I've been reading and re-reading your description of the problem and the only thing that comes to mind is a ground problem.
Have you got a volt meter? We should be able to measure where the problem is.
I don't think the problem is in the back. I think it's in the front.
When the lights and the turn signals are turned on, there is an increased current flow to the ground. If the ground is weak, they won't work. This could be the same ground as some of the engine electronics, which is causing the engine to stall.
I'm sorry but I don't have a picture of the grounds.
Your truck is old enough that GM dealerships still have paper manuals that you can look at and photocopy. It's also possible that your local library has GM manuals.
That's the best source. The Haynes manuals are about 75% accurate, too general.
Thank you for accepting.
I would really like to have a complete set of manuals. But they are about $2,000 per model year for GM manuals. Plus that's about 5 feet of books per year. It's just not in the budget and I don't have the space.
The newer manuals are online. I still prefer the paper books, but the online manual is more practical.