1980 Mercedes Benz 450sl How do I replace brake light switc

Remove the lower dash on the driver side of the car. It uses two 10mm bolts and a single 8mm bolt to hold it in place. Be careful as you're pulling it out of the way, because the footwell courtesy light is mounted to this. It's possible to disconnect it, but probably not something you want to do lightly.

Once the lower dash panel is down, the brake light switch is just barely visible. The most convenient way to access it is (yes, I'm being serious) by sitting upside down in the driver seat, with your head and arms in the footwell.