My dad recently passed away and left me his truck. He knew alot about cars and kept it in great shape. My tailights recently stoped working. The brake and reverse lights work though.

as usual...the main system ground is corroded...undo the lug off the MAIN ground connection and wirebrush it and the spot where it connects....this is that fat ole- Battery Cable...the black one...key to electrical health of all components~....get a NEW battery if this one is older than five years old~

fyi..gm has sections of harnesses running under the truck and when it gets back to the rear it splits right behind the drivers side rear wheel.. tail lights, brake lights, trailer hitch ect... not sure if its snowing where you are at but I have seen snow pile up back there and disconnect or corrode where they connect. hope this helps