Easy fix , but I needed a a way to reset and check to be sure so I purchased an Autel Maxicom ML808, which allows many functions to be accessed including airbag resets . The tool showed a high resistance in the driver shoulder belt tensioner.. out pops the rear drivers side panel Open the access panel (easy , remove the torx screw , pull the Rear boot weather strip and just pull it away from the body ) there are nice easy attachment bungs that hold it in. I unplugged the electric connectors (car keys out of ignition) and cleaned both sides of the plug with electric contact cleaner and dielectric grease on the ends , Re plugged and checked the tool again . The fault cleared . You could not see Any visible corrosion !!!, but obviously there was enough to throw out the values . WOW . Easy fix without going to a mechanic. I can only imagine that a Smart / Benz dealer would of just replaced the tensioner ($$$) my two cents .