I have experienced this with my 90. It ended up being the ground wire from battery to fender.
While your truck is running, go move wires around on the battery and see if you can get it cut out again.
I could literally be driving and push the horn and totally shut the whole truck down.
What’s...
I would also follow up on the tail light circuit boards. I had a very similar issue with my 98, all lights would work fine untill I turned on the running lights/headlights. Then I found a rear light frying the housing, I bout new tail light assemblies and everything was fine.