Under the seat is a small 2 pin connector adjacent to the main connector that houses the air bag stuff. Unplug the 2 pin connector only, see if the light still comes on, if it doesn't come on the problem is in the pressure switch in the seat. If it still comes on it's possible the car is trying to warn of a problem with the seatbelt pre-tensioners or the seat air-bags. Sometimes the warning lamps are multi-functional and this is not well documented in the manuals. I agree with Easy Rider, the quality of 'problem solving ability' or thinking outside the rhombus shown by some mechanics is woeful.