Number 1 20A fuse blows immediately when I press the brake pedal

The multi-function switch is not a good suspect. There may be a ground wire going to it, but even if there is, it is too far away to short to something else. More commonly, if two contacts remained connected at the wrong time, you would just have some light(s) that stay on when they should not.

A better way of 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 is 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.

Expect to see the test bulb become bright when you press the brake pedal. Use a stick between the seat and brake pedal to hold the pedal down while you look for the short. Look for chewed-up wires in a trailer wiring harness, screws run through wires behind tail light assemblies, melted sockets, and things like that. I have also found custom dual exhaust systems that ran too close to a harness and melted the wires above the rear axle.

If your rear turn signals use the same bulbs as for the brake lights, see if the signals work. If both do, you know those circuits are okay. That still leaves the center high-mount brake light.

If you cannot find anything that makes the short go away, I will find a wiring diagram to see if I can come up with more ideas of what is on that circuit.