If you want to solve it yourself you will need to buy a tester that plugs into the OBD port for your car, and read the fault codes. Depending on the car, these vary from about $200 to $600. Once you read the code, you an then dismantle the relevant part and fix the fault. Often it is a plug that is corroded and cleaning it sometimes will work. Under the seats is a common failure place as it get dirty there and also the seat slides back and forth. The reason it cost so much is that the code readers can cost a fair bit of money and you need the knowledge on how to use them, plus you must be handy with tools and able to resolve the problem. You also need to be VERY careful with airbag systems as you can trigger the airbag and hurt or even kill yourself, so make sure you do the research and understand exactly what it is you are doing. If you are not certain, then pay the money to get it done.