automatic seatbelt

My wife owns a '95 Escort LX wagon with these ridiculous automatic seatbelts (which i hate). Anyway, the passenger seatbelt works only ocassionally (thank god the driver side works). Strangely it works once no one is sitting in the seat and you open the door. Is there any sensor/switch in the seat that recognizes someone's sitting there? Or what else can it be that it wouldn't move the seatbelt forward.
Thanks for any advice.

i would say the motor between the passenger side and rear door--mine does the same thing its behind the cover its clicks alots-