FYI on a R129 sl. If your seat belt locks up on you and it won't pull out. before spending a lot of money on service and parts. the bowden cable may be stuck at the seat belt latch. use a screw driver or the opposite seat belt buckle to free it. push down on the good seat latch to see how it's supposed to work. if you listen carefully you can hear the cable ingaging the tensioner. The cable runs from the latch to the belt tensioner. when you hook the belt it pushes the cable up to the tensioner and locks it. so you may just need to clean the latch. If you have an air-compressor, you might try blowing the dirt out with high pressure nozzle and put a SMALL amount of sewing machine oil or zoom oil in the seat belt guides down inside the latch to free it up so it springs back where it is supposed to be. if that that doesn't solve the problem it can get fairly expensive replacing the tensioner or the belt latch. on any given sl the latch and bowden cable may or may not work as intended.