it's a switch that's the exact shape as the rear defroster and 4-way flasher switch.
on the cab it's located near the shifter in front of the window switches. on the coupe and sedan, I believe they put the switch in one of the 4 openings where the 4 ways and defroster live.
in any event, it should be a visible switch in the cockpit...
Wilf - Toronto, Ontario.
'99 E36 M3 Convertible
'88 E30 325i Convertible