Engine starts but quits when key released

I've got a strange one. My 73 Dart has a 440 and MSD 6A ignition. Up till today it started no problem. I was tracing some wires for my back up lights back to front and have gotten them working off the neutral safety switch. The old switch was disconnected and the relay was grounded to start. It seems like the power to the ignition is shut off after I release the key after the engine fires. Keep in mind that the existing wiring in this car is a bit of a rats nest from previous owners and there could be poor corrections anywhere. has anyone run across this problem and maybe have some trouble shoot ideas. This is my first post, but I have already benefitted from other s on the site. Great stuff