Car wont start unless door is open

Let's determine if you have 74 car (even though it may SAY 73) or maybe someone has swapped in the 74 harness. Look around under the hood for a box about the size of a horn relay with a push button reset. This normally will be on the left apron or left side firewall. If so, go here, read post 17 & 18

bypassing interlock system

Then since the problem has changed, start out like it is a whole new problem. Clip your meter to the yellow start wire on the starter relay. Whenever you twist the key to "start" you should have power there. If not look to that "path". From key switch ----"yellow" through the bulkhead---and to the relay. If it turns out you have the seat belt interlock, then the yellow wire goes through that