Reverse or back-up ligth switch...?

Hi. The group of ligths mounted on the rear boot lid, contains two small outer bulbs for the license plate ilumination, and one larger center bulb for indicating Reverse is engaged (correct?) . The center ligth is not coming on upon engaging reverse. I looked at it and the filament is intact. I measured the voltage between the center contact inside the socket and ground and there is none. I even disconnected the plug which brings the wires together and still no voltage "upsttream" of the plug. The only thing I could not do is run the engine and engage reverse and go back there to look, for obvious reasons since I was alone....lol. Does the ligth only comes on if engine is running and reverse engaged??? I cant believe this is the case. It must came on upon engagement + ignition key ON but engine not running...correct?
So, either the wire is damage somewhere else, or the switch indicating reverse is engaged is bad. I figure it must be a switch somewhere to indicate reverse is engaged so send voltage to the bulb in the rear to illuminate (back-up ligth..). Where is this switch located? does anyone knows? Thanks a bunch. Claudio.