I would submit that if removing the input from the back of the radio solved the problem, The root cause is not the radio. It is in whatever powers the radio. What work did you just have done? Dumb question, do your back up lights come on when you disengage the air brake? If it were me, I'd put a meter on the wire you cut and verify you have signal there (not sure if it would be 12v, 5v or ground).
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2015 SOB 5th Wheel
2013 Chevrolet 3500 CC, DD SRW
1 Wife, 2 Kids under 5
Camping nights: 2015 - 19, 2014 - 39, 2013 - 48