When I brought my car in to the MB mechanic on a non-related issued, I pointed this problem regarding the seat belt light remaining on all the time. When I picked up the car I did not think further about it until I parked the car in the garage. The problem seemed to have been resolved. I looked down at the storage tray between the seats to find 2 lightbulbs for the ovehead light (seatbelts only). The solution seems to be that simple - remove the bulbs. This does not affect the operation of the courtesy lights.

Hope this will end your frustation.