99 Mercury Grand Marquis why is my brake light switch wiring heating up and melting when i press the brake pedal

You've got a short somewhere. Start with the stoplight bulbs, remove them and look for problems, and clean socket and contacts. Do the brake lights come on? If that's OK, then check wiring going to the bulbs in the trunk, maybe a retainer clip has worn through the insulation on a wire. If still nothing, check the stoplight on the rear package tray(rear window). Same situation with it. Check socket, wiring. Any new speakers installed in rear? Check all wiring inside of trunk. Can you take it out of park when hitting the brake? Maybe the wires going to shift interlock on the transmission.