Baron,
I might very well be wrong--but this might be the power connection for the rear-window defogger on the hard top. Do you have a hard top? If so, check and see, you might have a pigtail wire coming down for that.
Michael Salemi
1969 280SL
Signal Red 568G w/Black Leather (Restored)
President, International Stars Section
Mercedes-Benz Club of America