[tbucketnut]

Hello, the best way around this problem is to scour the bone yards and ebay looking for a top cover from a 1967 Sm420. It has a factory backup switch incorporated in it, not the cheasy dealer add on that found their way into many 60-66 Chev/GMCs. Unfortunately the 1967 sm420 was the last year put into trucks so it will be a rare find but they're out there. I came across one last year that will be going into my 64. Slow load on the picture but worth the wait.
Steve