As far as I'm aware the 2000 Model year Mustang does not integrate the seat belt minder or butt in seat switches to the air bag system. I suspect that something happen during the work.
But it help to know IF the seat belt minder works.
If asking for an educated guess you have damaged ground G400 during the work. Recommend testing the ground of the seat belt buckle to confirm it's able to carry a current. Use a test light that will "load" the circuit. With the test light pig tail on battery positive the test light should light bright when a good ground is probed. Always test the test light using the battery's negative.
2003 GT Convertible (sold & missed)
2000 GT coupe (Craigslist project. Fixed. Now my DD). Windsor to Romeo swap.