The set up Grigg is talking about shows up in my 1957 Chevrolet Accessory Installation Manual. The switch attaches to a new gear shift lever retaining cap and the light switch lever attaches to the gear shift shaft. When the trans is put in reverse the switch lever pushes down on the switch button turning on the lights. The part number is 987454. I'm not sure what year it was introduced, but it would work on ALL SM-420 transmissions.
Mike B
