why are my 2001 mercedes c240 break lights staying on

see if you can locate the brake light switch up above the foot pedal,usually at the area of end it is a micro switch has a electrical connection so when you push down on the pedal,it closes a small button,sending power to the lights the switch maybe stuck in,or out of adjustment

brake light switch, try removing it and cleaning if not new 1, test with volt meter the polarity when pushing in the little peepie and releasing it

thanks guys - i got a new switch and replaced it - now brake lights work but only when key is turned on - is that correct - should the brake lights work all the time?