The seat sensor would be a good place to start. I would also look at the harness under the seat for any damage.
It may be cheaper for you to just get a seat from an auto recycler/wrecker. You can get a matching seat with OCS and airbag for $200 or less. .The OCS may be messed up, an expensive weight computer that weighs the passenger to determine the airbag detonation amount for adult or child there. OCS is the 'occupant classification system', there is a computer under the passenger seat hooked to a pad with weight sensors in seat bottom; that setup is part of the seat and $$$$$$$$$$ changing it.