The passenger-side seat belt retractor on my 1994 E320 Cabriolet has a mind of its own. Left unbuckled (and with no passenger or weight on the seat) the seat belt retractor suddenly starts operating repeatedly, in and out rapidly. My temporary "cure" has been to keep the seatbelt buckled into place. I assume there is a switch or sensor that is awry. Can anyone help me locate it or tell me how to replace it?

Thanks!