the only way you can disable a passenger air bag on a frontera is by replacing the airbag ECU (electronic control unit) which itself cost £500
the new unit then needs to be programmed not to recognise the passenger air bag which can only be done by a dealer.
this action in itself raises many legal issues and many dealers would advise against such an action as you are changing the original safety specifications of the car.
most dealers will request a letter from you, your insurance company and a solicitor exonerating them from any consequenses should an accident occur.
there are then the legal consequences to yourself should you sell on the vehicle in a non standard condition
honest advice -- leave well alone
robb