SLK/R170: Crazy Electrical Issue
but it sounds to me that you've got a short between what ever shares the same wiring as the brake light's and interior lights
im not a professional when it comes to isolating electrical gremlins on these vehicles but i'd be starting with removing brake light globes one by one and testing the brakes to see if the interior lights will stop triggering
hopefully it's one of the globes and you can focus more on that




