Ok here it is... I have a 1986 s10 that i purchased non-running. The head gasket was blown. After replacing the head and got it running good other issues raised up. Everything works good when the headlights and parking lights are off except the radio (goes in and out as if it loses all power) and it goes out if i use turn signals. When I turn the headlight switch on everything goes haywire. Gas gauge drops to empty, turn signal indicators come on and stay on, radio goes out, no dash lamps, and when my low beams are on the dash indicator says my brights are on. when i switch to my brights the highbeam indicator goes out. When i'm driving down the road with headlights on it will flicker to normal then back to all this. I have checked all fuses and looked for a ground wire that i may have forgotten and have found nothing and unplugged the radio to see if it was the problem and no change.

 

all exterior lights work as they should

 

I'm having very similar problems. Did you figure out what the problem was?