Seat belt switch

Here, try this:

http://www.forabodiesonly.com/mopar/showthread.php?t=14304&highlight=interlock

or dig into the harness, download the 72 service manual, and find the yellow start wire from the bulkhead connector, and be sure it's wired to the start relay

If you are getting power "in start" to the yellow wire (push on) at the start relay, check for trouble at the neutral safety switch. To start that out, ground the OTHER start relay terminal.