I'd check the switch first.
Pull out the carpet under the armrest and remove the single screw. Remove the plastic rim around the gear shifter and pull up the wood panel. The window switches can by removed just by pulling them out. You can swap right/left switches to see if that's the problem.
The switch is very likely to be the problem, IMHO. You'll probably find all kinds of gunk down there (crumbs, spills, etc.) that could cause it to fail. In my 90 300E I just opened up the bad switch, cleaned the contacts with fine sandpaper, and reinstalled.
Hope that helps.