There was a switch and a bracket that mounted to the shift handle, and made contact only when the trans was in reverse. It's a pretty rare item, IIRC. I've seen OEM parts list that show the 4-speed switch and bracket, but I've never actually seen one in use.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!