When you take off the door pannel you will see who plugs which plug into the seatbelt mechanism (the one bolted to the door.) Do these plugs do anything other than turn the seatbelt light off and on.

I'm worried they might, since I can't get the shoulder strap to lock up anymore when I tug on it.

I thought the locking mechanism on the seatbelts was purely mechanical...

It would definately be silly to have it not be mechanical, since if there was a short the seatbelt could fail.

I'm think the plugs don't have anything to do with the seatbelt light, because there is a connection for that one under the seat which hooks to the seatbelt buckle.

It turns out that the seat belts do have to be plugged in to work properly. The should restraints only work when they are plugged in and you are stepping on the brakes.