OK. In '99 and '00, the passenger seat sensor is a two-wire white plug and all it does is illuminate the Fasten seat belt light for the passenger if there is a butt present and buckle not closed. No connection to SRS.
In '01 and '02 it is a four-pin plug with three wires. The SRS system has a separate connection to the seat sensor and they use a common ground.
For the OP, on the truckside, the blue-yellow wire is for the SRS. The yellow-green wire is for the warning light. There should also be a white-black for ground.The seat should be a yellow-green for the combined seat/buckle sensor plus a white-black for ground.
The diagram isn't clear, but I assume the SRS tap is before the buckle switch as I believe the SRS should fire whether or not the belt is fastened. Unfortunately, the early seat will not have this tap. As far as I can tell, the early SRS does not care if someone is in the seat. Only '01 and '02 have this.
I think the best thing for the OP to do would be connect yellow-green to yellow-green for the warning light and leave the SRS thinking there is always a butt in the passenger seat. However, I can't be sure if that is an open connection or a grounded connection on the blue-yellow circuit. If you had the old seat, you could test it. Otherwise, we would need someone with an '01 or '02 to test their seat to see how that blue-yellow circuit is switched. I would not want to guess if a passenger's life might depend on this. Of course, safest would be to get a compatible '01-'02 seat.
I'd test mine but I have a '99 so no help. Sorry for my earlier post which I will delete.