The supply of original Pantera backup light switches dried up a little while ago, and in desperation the vendors substituted something else, which mechanically worked just as well. However, it required a special connector to hook it up electrically, and of course they were selling just the switch, but not the corresponding connector!
I believe the situation has been resolved with the original switch now back in production. (It never ceases to amaze me that a given Pantera part is usually only out of production for a few months before one of the Pantera vendors will tool up to reproduce it, or buy a batch from the OEM manufacturer who still has the original tooling).
The switch's function is quite simple--it just has a plunger which is acted upon by the shift linkage inside the shifter box. The switch is threaded into the cover, and secured in place with a locknut; threading the switch in or out alters the engagement. Thread it in too far, and the switch will prevent the linkage from going in reverse; too little, and the switch won't close and the lights won't illuminate.
I can't remember for sure, but I *think* Bobby Byars of Precision Performance is the guy who had a new stack of backup light switches.
www.precisionperformance.com gives you all their contact info. When you give him a call, tell him I sent ya!
Ask him if he has the 'new-style' or 'old-style' switches. If he only has the new ones, make sure he can supply the corresponding connector for the car's wiring harness. Otherwise, keep looking....