Guess what this is...

Definately not aftermarket ( it actually has the Pentastar and part #). I have it in my Duster. The Duster is a 72, however the harness is from a 74. No seatbelts hooked up and starts fine. Don't believe it's a seatbelt thing. Could be wrong but mine starts fine without seatbelts hooked up.

Do you have the part number? With that and a little research maybe we can find out what it is.