The odd sounding thing here is that you are getting a code for a passenger seat airbag when the 1996 chassis (AFAIK) didn't HAVE a passenger side airbag. That came out with the new engine/dash/etc. in 1997.
So it seems like either your airbag system is confused for some reason, or.... I wonder if components from a later/different van were ever installed? When I was reading the service bulletin about removing a passenger seat (in a 1997+ with a passenger side airbag), there was information in it about the pre-tensioners, etc. being coded to the vehicle. Obviously there must be a procedure for replacing the originals with new ones, but I wonder if there could be a case where new parts were put in and not "registered" correctly (?).
I know very little about the system, really, so just take this as some fireside "noodling."
PS: In case it would help, the link to the Ford document about removing a seat (which is for 1997+ but did also have general information about handling pre-tensioners, etc.) is in a thread I started called something like "So can I just remove the passenger seat?"
PPS: Here is a link to that thread. (OP I know you have seen it, but since others may read later...)