The AMX and Javelins used a plunger switch that was located on the 4 speed linkage. The exact location is where the reverse rod is located there should be a hole in the shifter housing and when you throw into reverse it will engage a plunger switch.
To replace these switches can be an expensive proposition in that the after market guys are getting about $85-95 for them. The AMX we have doesn't have one there anymore and the wires are taped up until I can find a replacement for it that doesn't cost so much.