Stangfamily

I recently had my 71 coupe in for tranny work (rebuild). Now that it's back it won't start any more. The weirdest thing is that when you turn the key nothing happens but when you make contact with the gas pedal you engage the radio and starter! That lasted for a few trys and now nothing happens except you still get your dash board idiot lights to come on. Have checked wiring bundles and they appear ok. Changed out starter relay and no change. Followed gas pedal and cable through fire wall and don't see anything. Touching the pedal must complete a ground but wanted to know if this has ever happened to any of you? Hope you can help.