I have downloaded and printed the '72 A-body (Dart or Valiant) wiring diagram found in many places online, and it has a significant drawing error.

The Start line from the ignition switch is shown as running to bulkhead connector pin L... which in the engine compartment is connected to the horn! Likewise, the horn relay line is shown running to pin T, which is the line to the starter relay. Both are dark green with a red stripe, which is probably what led to the drafting mistake.

If your A-body were really wired this way it'd be nearly theftproof - push the horn button to start the car, turn the key to blow the horn

Anyone sufficiently versed in reading schematics would probably notice this and not attempt to wire it that way! But just in case, I thought I'd share...

I noted a few mistakes in the 72 FSM when I had my Swinger, but that wasn't one of them I noticed. I had an issue with something draining my battery over night and the fuse panel wiring was not even close to my car's wiring. Only thing I could figure, at the time, was that my 72 was built very late in the 72 MY and might have had 73 wiring, or there was a running change of some sort. Never had a 73, A body or FSM, to compare so I never sorted out the issue, but I did fix the drain (trunk light).