2000 Mercedes Benz C230 Brake lights staying on

I am having a brake light issue. The brake lights (driver, passenger AND center) all stay on constantly. We got this corrected for a very short time when we replaced the brake light bulbs in the driver and passenger sides - they seemed to be the wrong or mismatched bulbs, so after replacing and checking them visually, thought we were all good. However, today the problem reappeared. Besides being totally unsafe to not have working brake lights, the lights have gotten so hot from staying on that they have begun to melt the tail light covers. I would appreciate any advice! Thanks!