Originally Posted by jage
Not sure about 96, but my SMB has a sensor in the seat and a seatbelt connected sensor. If the seatbelt is separate, there might be a "is there a person in the seat" sensor that isn't connected (s.t. the airbag doesn't fire on an empty seat).
Or you could have newspaper stuffed where the airbags should be.
Seatbelt integration with the Airbag system started in '97 as far as I know. It slows the airbag if the seatbelt is worn, since the airbag has smaller role in that case, and the seat sensor also affects airbag force. The wire to the seatbelt is what triggers the "pre-tensionser" (again, '97+). It's also a small explosion, but it pulls the seatbelt latch down to remove slack.
Unless Ford upgraded the restraint system in some late '96 vans (possible) the seatbelt sensor is completely independent.
Note: Ford did install the 460 into some '97 model year vans which were built in '96. Any chance the OP's van is actually a '97?