If the seat belts are working, I doubt the sensors are the problem. The two things that can set off the light are the controller itself if something were wrong and the shoulder belts, If they both aren't buckled the light will come on (it actually senses how far the belt is retracted not if it's buckled but thats the easiest way to explain it)
But if they are buckled it never hurts to check the sensors. They are little back boxes with 2 wires coming out at either end of the track. To access them you will have to pull the rear panels and the trim above the door. Once you have all the screws out (make sure to get the one hidden in the back wind shield behind the seatbelt covers) there are clips below the panel to access the thumbdrive for the motor (little square panle that pops out) that must be pryed off then the rest of it pops off (if you have lights in the rear panels unplug them). It's kind of a pain but there are no shortcuts so...