Read the airbag fault codes. It could be the seat belt tensioner.

 

It will most probably be the passenger occupancy mat. These aren't cheap to fix for the mat plus fitting, which is a bit tricky. I bought a plug from ebay for £45 that fools the car into thinking that someone is always in the seat, and thus rectifying the airbag light coming on.

 

Things that cause airbag light to be on.

• Seatbelt tensioner
• Occupancy Mat
• Seat sensor
• Faulty Airbag or wiring

Disconnect your seat plugs without disconnecting your battery first.

 

This link has more answers to the airbag light problem.

https://www.e46fanatics.com/threads/565806/