I'd suspect contact problems as well. Either in the plug or somewhere in the wiring. Driving the vehicle causes vibrations that can make an "iffy" connection work again. Could also be questionable solder joints in your "convenience module" wherever it may be.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]