Look on the driver's side wheel well for a cylinder, about 1.5 inches in diameter and 2 inches high. Unscrew the top part from the car. You'll have a bell shaped part with a plug in it. Take the plug out of the bell shaped part (this can be hard...). Put a piece of foam or even a cigarette butt in the bell housing and put the plug back in. Screw it back on to the socket on the wheel well and try it.
Worked on mine and many others...
1982 300D - <A HREF="http://members.xoom.com/nicodore/