could be many things... may be the door switches.
not sure of your skill level so maybe the best thing to do is get familiar with the wiring diagrams of your truck? particularly with the inside of the cab...
you can go here:
http://www.2heeldrive.com/Chevrolet/...ng_Diagrams%2F
if your's is an 86 it is probably in 81up Chevy directory
http://www.2heeldrive.com/Chevrolet/...F81up+Chevy%2F
some of these are huge files, so it would be best to download them to your computer then open them up for easier viewing.
Not sure but maybe you could start with the file of the complete wiring manual 1986_GM_CK_Light_Truck_Wiring_Manual_S.pdf
http://www.2heeldrive.com/Chevrolet/...g_Manual_S.pdf
that's my suggestion, there will be others who will help
good luck