Brake light fuse keeps blowing but found something

Sounds like you're headed in the wrong direction. You already identified the left rear wiring harness as the source of the problem, so why are you going into the dash area?

A simple trick to finding a short is to replace the blown fuse with a pair of spade terminals, then use small jumper wires to connect them to a 12 volt light bulb. A brake light bulb works well. When the circuit is live and the short is present, the bulb will be full brightness and hot so be sure it's not laying on the carpet or against a plastic door panel. Now you can unplug electrical connectors and move things around to see what makes the short go away. When it does, the bulb will get dim or go out.

Check if there's a trailer harness. Those commonly get chewed-up wires that can short together.

Yup. The bulb limits current to a safe level, about one amp, and it gives a nice visual indication of how much current is flowing. You should see that test bulb get bright when you have the harness plugged in and the circuit is turned on. That most likely means having a helper press the brake pedal or you can put a stick between the seat cushion and brake pedal to hold the pedal down. The bulb will get dim when you unplug that harness. Also, you'll see the working lights start to glow very dimly. They won't be full brightness because that test bulb is dropping some of the voltage.

The lights are making a squealing noise? What sensor are you referring to?

Unplug it and see if the short goes away. I never had one of those apart so I don't know what's in there or how they work.

Can't tell you that, but I can give you some generalizations. On many newer cars the rear turn signal and the brake light are totally different bulbs. That makes connecting trailer wiring very complicated for car owners but it simplifies the signal switch, which is all the manufacturers care about. With those, all three brake lights will be wired together and will have a single fuse for the entire circuit. On older cars like yours, and everything I drive, the same bulb is used for the brake light and the turn signal. The two left and right signal / brake lights need to be on separate circuits, and both of them are separate from the center brake light. Turn signals feed from the flasher which usually has its own fuse, and the center brake light will have IT'S own fuse, ... Usually.

The center brake light would still be protected if it only had the brake light fuse. The clue there would be which fuse(s) blow. If the wire going to the center brake light was grounded, creating a dead short, it would blow its fuse or the brake light circuit fuse, but not the signal fuse. If the right brake / signal wire was grounded, that would blow the brake light fuse when the brake pedal was pressed and the signal fuse when the right signal was turned on, but not when the left signal was turned on.