So if the seatbelt light is run by the belt release on the seat... and the door ajar light is run by the switch in the door jam (on the body of the car)...

...then what are the wires on the door harness for? (The speaker wires are obvious, but what about the ones that run to both seatbelt assemblies and to the door latch assembly?)

NOTE: I do not have the power windows, heated mirrors or anything else electrical on the doors.

 

Two automatic seat belt re-tractors are the main players. The door latch wire energizes the seat belt re-tractor solenoids to prevent seat belts from locking when door is opened. I can go into more depth if required

 

Perfect, thanks that's all I wanted to know. Can I ask you one other thing you might be able to help me with?

When I removed the seatbelt assemblies, they must have retracted too much because now I can't unroll them even while manually activating the release. Is there a trick to unbind them again?