We have a winner - almost
This isn't the kind of reverse switch that bolts up to your speedo and senses when the car is rolling backwards, this is the switch which bolts up to your gear shift selector rod and turns on as soon as reverse gear is selected.
Have been looking for one of these for a bit!
This is a photo of the other kind of reverse switch you could get - these Tachowellenschalters from Hella were a very basic version which goes in-line between your speedo and your speedo cable. Still very effective though and a nice accessory to have!