LouieHenry,
Some of the late model 280SL's had a first aid kit mounted in the back of the trunk, But there wouldn't be a wire going to it. Given your other posts about amps and speakers, I bet someone mounted an amp back there, and since you subsequently said that they removed an amp from the car, i bet that's it. I bet the wire is your power supply, but that's a guess.
Mike Mizesko
Columbus, OH