Passenger airbag sensor may be defective, quoted 800 repair 626 98

Does anyone know a cure for this warning light problem, the previous owner says a main dealer quoted 800 for repair as the sensor is buried in the
passenger seat. I bought this 98 Mazda 626 on ebay and apart from having to have new rear shocks to get through MOT, this faulty sensor repeatedly flashing and a tennis ball size dent in the rear this GSi Estate is very reassuring to drive, despite 104k miles on purchase.
Any suggestions on how to stop the warning light flashing, apart from paying out several hundred pounds. As you will be aware my knowledge of car servicing is fairly small, if not completely absent.
Thanks for any help!
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