my radio automatically comes even when the engine is off. just started this

I had the same problem with my 1999 E320 wagon. Got another radio on ebay. It worked fine for 2 months and last night, the radio came on again while the car was off. I had read it was an issue in the volume control knob and could be fixed, but, right now, I can't seem to find where I read that. Possibly youtube. I'll keep looking and will let you know if I find anything. Meanwhile, remove the fuse. Under the back seat, passenger side, fuse #5. Unfortunately, you won't be able to lock your car with this fuse removed, but, it's better then your battery going dead. Trust me: The radio will kill the battery. Please share if you find a solution. Now I have 2 radios to fix!