1995 gmc Sierra lighting problem

This weekend I will be helping a friend with his truck and I'm just looking for direction to find the issue

"Right turn signal works fine then the left makes all the lights blink as well as the dash lights also happens with the hazards"

That's all the info I got from him. I haven't done any automotive work in years and decided to help out and hoping for some insight hoping someone has had a similar prob.

From what research I did I found that a plug in trailer wiring could cause issues and I will be wiggling this and that to find a short etc etc.

TIA y'all

Check for shorts in light sockets, the light wiring, and even within the bulbs themselves. I actially had a failed/shorted bulb do that same kind of thing on a GM van.

Rear tail/turn housings are a known issue with poor solder work.

Check for shorts in light sockets, the light wiring, and even within the bulbs themselves. I actially had a failed/shorted bulb do that same kind of thing on a GM van.
We started looking over the truck and began to quickly realize that it was severely neglected by 1 of the previous owners and we think at least 1 of them was a kid due to the immature up keep and rattle can painted interior as well as added on wiring for previous aftermarket lights and whatever else. My buddy thought it was the switch on the column but that wasn't it.

He started replacing bulbs when we noticed there was different types of aftermarket bulbs all through the truck, and at least 1 bulb socket was corroded. After replacing all the bulbs we noticed the lights were brighter AND the problem was fixed!!!