MAKE SURE YOU HAVE NOTHING IN YOU PASS SEAT OVER 5LBS. BUT LESS THAN 60LBS. (approx) I HAVE HAD CUSTOMERS WITH HEAVY PURSES OR BRIEFCASES MAKE THIS SAME COMPLAINT. THE VEHICLE WILL DISABLE THE PASS. SIDE AIR BAG IF THE OCCUPANT IS TO LIGHT (eg. child) THIS WILL TRIGGER THE LIGHT ON THE DASH. GOOD LUCK.
Probably a long shot but is your car still under factory warranty by mileage? If so it may be a free repair.
The airbag light is on because the srs module has detected a problem somewhere in the srs system (supplemental restraint system). It includes crash sensors, control module, airbags themselves, seat belt pretensioners, any active head restraint system, etc.
The most common srs failure is the "clock spring" located inside the steering column. This is a taped wheel mechanism that allows wires to stay connected to the steering wheel. The electronics that run through the "clock spring" are for the drivers side airbag, the horn, and any cruise control or radio controls mounted on the steering wheel.
When this "clock spring" breaks, it may interrupt communication between the drivers side airbag and the srs module. Try and see if the horn, cruise control and radio controls work before bringing the car in for diagnosis. If anything there does not work make sure you tell your shop in hopes to keep the diagnostic part down some.
SRS repairs are usually costly, expensive on cars today is relative though.
Best of luck!