The internal contacts are dirty. You can spray contact cleaner into it after removing the knobs (they pull or pry off -- watch that you don't lose the headrest one) and the face plate on the door (I think it just pries off). Flood the inside with spray contact cleaner (safe for plastics, please!) and work the switch and it should be good to go.

If not, take the switch out of the door, unplug everything, remove the screw in the back, pry up the clips that hold the front on, and VERY CAREFULLY lift the front off from the back of the switch with the front facing up. It's full of tiny bits, you don't want them to go jumping off the table, believe me!.

Inside there are about six pairs of rockers with a pair of ball bearings top and bottom with a spring in between. These rockers have pivots that fit into slots, and when rocked over press a wing-shaped contact onto a stationary one. You can either gently spray them down until they are all clean (center contact and the one on each end, plus the stationary ones) or clean them with a new pencil eraser. Put it all back together, making sure there is a ball bearing on top and bottom of each spring, slip the front back on and press down to clip the clips back on, then install the screw. Should work fine then. Gently shake before re-installing -- if there are loose bits rattling around, take it apart again and get it all sorted and re-assemble.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!