Q: Burnt wires leading to tail lights.

My 98 k2500 pickup has a lot of issues that i'm fixing one at a time. Well, the tail lights, rear signals and reverse lights weren't working. My friend messed around with the rat's nest of wires in the back and even swapped out a new rear harness from spare parts. No change. So he messed with the wires again and re connected a few grounds and the lights looked like a pink floyd light show. He finally got the lights working properly, except the reverse lights would only come on for a second when I start or stop the engine. The next day I brought it over to him and we followed the cluster of wires from the back to the front and where it goes up into the engine compartment, the wires are all melted and the copper is touching. I tried disconnecting the 3 connections under the hood to get access to the crispy wires and noticed that my fuel pump wasn't working while they were disconnected. We connected the harnesses and checked the lights and now the reverse lights are stuck on. I've looked

My car has 172000 miles.
My car has an automatic transmission.