I have a 1972 gmc 2500 4x4 truck tail lights and brake lights stopped working changed fuse and it fryed ground wire to rear harness got new rear harness changed fuse again same thing the ground wire heats up imeadately not sure why all of a sudden that's happ please help

You have a short circuit someplace Look for damaged or pinched wiring.. also check your tal lights and make sure a leaking seal doesn't have them full of water.

Look inside the socket to see if contacts are touching. Wires can be short where melted as mentioned by smoothy.