Have a 72 duster the car dies when l let go of switch.however did replace the ballast resistor,usually that's common with bad resistor.the car still dies after replacing resistor.the ignition switch bad to l suppose?

The brown wire coming out of the ignition switch gets power during "start". Once the engine starts and you let off the key, power transfers to the blue wire coming out of the ignition switch. You have a problem with the blue wire(run circuit) somewhere. Whether that's the ignition switch, a bad wire or a bad connection. Start testing the blue wire for power at the ignition switch and work your way into the engine compartment. My guess is it's a bad connection on the steering column connector or bulkhead connector.

Good troubleshooting advice above. I would add that it is possible that your replacement ballast resistor is also bad. A simple test light is all you need to track down this problem.

Go here for wiring diagrams if needed. https://mymopar.com/wiring-and-electrical/

Bulkhead connector. They go bad. Probably a melted terminal connection. I bypassed mine with a separate pass-thru wire. The factory method was fine for 10-20 years, not so fine for the LONG haul!

look real close at the steering column to main harness connector[s].
i have repaired many over the years due to the terminals becoming loose, resulting in the connectors becoming "melty".
had a buddy with a 72 duster buy a new ignition switch because of this. he installed it, and had the same problem.
as i suspected, that connection was not making a tight contact between the terminals. tightened all but one, which needed replaced [i forget why, but it was an easy replacement], and all has been well these past 5 years or so. he has a spare ignition switch on his shelf.

I would jump ballast resistor then all above

Wires backward on the ballast resistor !

Tried jumpin resistor didn't keep the car running on its own still

Tried other good resistors then brand new one,didnt fix the problem. Thanks for showin where wiring diagrams are at.

Send me a pic of how wires should be on resistor? I have a single wire one.check wiring it was right on resistor.

Single wire resistor doesn't matter

As said I would start at the bulkhead & work downstream toward the ign switch or steering column harness then work back to the 2nd terminal in the bulkhead for the open.

Last time I had this happen it was one of the fusible links just ahead of the bulkhead connecter on the fire wall...