My '91 has the "passive" system that was required for one year. The shoulder belts have a motor that puts them forward and back along the top of the door frame. If you have that system, both shoulder belts must be engaged to the latches and the motors have to pull them all the way back to the latched position or the light will stay on.

Otherwise it's probably the switch in the buckle. I would think it's a simple on off switch with no trick resistors so would be easy to check with a meter.

 

mine are regular but on the door. ill keep checking