I just bought this 89 Silverado 4x4 that had many problems. None of the signal/corner/interior lights worked among other things not important right now. I went through all the fuses and found 2 blown fuses that had copper wound around the pins in order to keep them hot (idiot). when I saw this, I immediately disconnected the battery and begun searching for shorts. I found about 10 melted wires and a direct short at the trailer connection. After replacing all these, and fuses, nothing still worked. I replaced the 2 flasher modules and now the blinkers and interior lights work. The weird thing is now that whenever I put the "stop" fuse in, the brake lights come on wether I have the hazard flasher in or not and wether I have the brake pedal switch connected or not. Could someone please shed some light.

 

Try disconecting the switch at the proportioning valve????? Good Luck.

 

most lighting problems i have seen in trucks is due to the trailer lighting harness and module. i would certainly start there by removing it and starting over. good luck!