Car wont start unless door is open

The only year that is SUPPOSED to have the seat belt interlock is 74, easy to identify---look along the driver side fender apron for a small box about size of horn relay with a reset button. But that does not work as the OP describes. It "trips" and opens the starter relay yellow actuator wire from the key, so it just won't crank until the button is reset---or the owner (wiring) bypasses the reset

I cannot think off hand what in the world would inter--connect the courtesy/ dome/ buzzer to disable the start relay

I would start by taking a good look under the hood at the wires going from the start relay to the bulkhead, and then access the wiring of the ignition switch connector leaving the column--look for any signs of tampering, splicing, etc,

Look for any added non-OEM devices/ boxes both under hood and under dash

74 interlock reset:

1974 duster seat belt interlock module removal

bypassing interlock system

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