I have a 1999 SLK 230. Left it in the garage for 2 days, then battery was completely dead (no sound, nothing). Removed connections from battery and charged overnight. Started fine the next day, drove it to work a day later. When I came out of work, it was completely dead again. Got a jump start and when the jumper cables were connected to the battery, the parking lights and fog lights came on (before starting the car, and the switch was off). Started the car and drove home, but could never get the fog/parking lights to go off. Needless to say, it's dead again. Any ideas?

 

Interesting.
Have you disconnected the battery again and determined whether after a few mins wait the lights remain ON after reconnecting the battery.

I suspect you problem is a simple dead battery and "confused" body electrics ECU.
What voltage do you have across the battery with
Ign off.
Ign on (engine off) and headlamps on,
and with the engine at say 2000rpm?

If the voltage goes below 12.5v the battery is scrap and although it can be recharged you will never be able to rely on it starting the car when you need it most!!

Stuart

 

I tend to agree with Stuart that your weak battery is probably messing with the electronics. My coupe had a very weak battery last winter and was throwing out all kinds of weird things like "light sensor damage", "visit shop immediately", "left headlamp out" or words to that affect.
That said, at some point many moons ago, the headlamp switch in my coupe was replaced under warranty. Whats interesting is that I have headlamp sensors and always-on daytime running lights and rarely turned on the switch unless I was going to need the high beam control. What was not functioning was the 'left side parking lamps' so the switch can fail.
My bet though is the battery.