Do download the wiring diagram from Advanced. It's the best available. Trying to follow the one in the manual is an exercise in frustration.
The purple circuit, you will see, powers a number of functions; the problem could be anywhere among them. It's also possible that the PO disconnected the part that powers the seat belt warning system, simply because it's annoying. It might be worth the investment of a fuse to hook it up again and see what happens. If no fuse blows, you're home free.
Assuming that the fuse blows: If I were doing this, I'd disconnect the two wires, then reconnect one wire at a time to see which circuit is blowing the fuse. Then, follow the wire enough to determine what it powers, and disconnect the various parts of that circuit and see what makes the short go away. At that point, you've located the problem, and you can just look at the offending component carefully to see exactly what the problem is. Likely to be something that just didn't have enough clearance between a terminal or wire and ground, and it's making light contact.
This is basic electrical troubleshooting. Just an application of logic.