try the trunk light. the pin switch may fail leaving the light on all the time.
I've seen it happen before. not sure how common it is though.
Otherwise I would investigate obvious things first, like the antenna steve mentioned.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.