This is a bit hard to figure. Typical problems are (in no order)
Bad or loose/ worn ignition switch
Bad ignition switch connector
Bad connection(s) in the bulkhead/ firewall connnector
Bad connections at the ammeter or trouble in the ammeter
Iffy intermittent start relay
Iffy intermittent neutral switch or connector
AND THEN THERE IS
The seat belt interlock, which was 74 only, and I've heard some early 75 models
[URL]https://www.forabodiesonly.com/mopar/threads/bypassing-interlock-system.14304/[/URL]
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This is a small box like a horn relay over on the driver side fender apron, and has a red reset button out of the top of the box. What you need to do is take the two yellow/ yellow-black stripe wires and splice them together.
Also as a quick check, hold the key in start while cycling the shift lever from park to neutral. If you hear a click or buzz, you are on the right track, either bad NSS or connections/ wiring, or just linkage out of adjustment.