Mine is a 2wd sapphire so maybe different but it's on the inlet side of the engine for definite. I think from memory, it's against the bulkhead by the corner of the battery tray? If not, it's a part of the engine loom. That sounds odd but to explain, I'm just about finishing re-building mine from a stripped rolling shell, and I used my old engine loom from my prior cossie. Before re-fitting it I had the loom tested and re-wrapped in different colour tape, and I clearly remember the reverse switch wire being re-wrapped in the same colour. And it was only the engine loom and one earth strap I replaced so therefore it is definitely either part of the engine loom (by that I mean the wiring that goes through the large rubber grommet in the bulkhead, that the ECU plug goes through, as I removed the loom complete with grommet) or up by the battery tray as a seperate wire to the loom.
Hope that makes sense after all that! Have tried to find a pic showing it but no luck yet.
cheeRS