On a manual tranny that switch is on the transmission. IIRC it's on the RH side, above the center line and near the front. Two wires to it, BBBL-Y from the fuse panel, SB (Swedish for black) to the rear of the car. This a very good place to check, as the connectors there are in a hostile environment.

On and automatic tranny the switch is part of the neutral-park switch, located in the shifter well. Wire from fuse is Y, out to the rear is SB. Known to go out of adjustment.

Note that if you have an autobox and the shifter detents aren't lining up right with the tranny detents - you'll know if this is the case - it means that the motor/tranny mounts have failed.

This can allow the tranny to be in reverse, but the shifter to stay a little out of reverse so the switch for the lights remains open.

Good Luck,

Bob

:>)