Backup lights were not available on SM-420 transmissions until somewhere around 1967. '67 was the last year for civilian SM-420s, with military models available into the early '80s.
The '67 model backup lights used an actuator (made from an alloy of unobtanium and hen's teeth) that bolted to the shift lever and a switch that bolted to the top cover, all hidden under a large rubber boot.
Novak does a $100 mod to the top cover that drills and taps the tunnel that the reverse shift rod slides in so a switch can be threaded into the cover that the shift rod activates. I talked with the tech at Novak that does the work and have included his instructions below. They charge $19 for the switch if you want to do the work yourself.
I plan to just mount a manually activated switch in the under-dash ABS plastic channel where my Vintage Air A/C controls and alternator idiot light are mounted.
Last edited by Bill Hanlon; 01/14/20249:38 PM.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12 '52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
