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I just did a total rebuild of my 1970 Formula 400 Firebird and cant figure out what this plug was for that is in the side of the transmission. If its the back up switch plug I cant find one anywhere. Any help would be greatly appreciated.
When your car was new, I believe that switch detected when you are in 4th gear. It then operates a solenoid that applies vacuum to your distributor vacuum advance can.
As for the reverse backup switch. On your transmission, you should have a switch and a small link that attaches to the reverse shift lever. The reverse lever is the rearmost of the three levers. This pic is from my '73 Corvette. You can probably get a backup light switch assembly for your car from one of the many restoration companies. You can see the photo below shows the same switch on my Muncie 3/4 shift lever.
09-13-2017, 09:43 AM
Muncie M22 Back Up LIght Switch Help 
