seatbelt help

[gatorblue92]

For about the last 10-15 years the rear driver side seatbelt in my 240D has had an issue extending. Whenever you attempt to pull it out it will lock up. I haven't ever fooled with it until now since I haven't needed both rear belts but with the impending arrival of son #2 i will need it for a car seat. Has anyone had any success repairing the seatbelt or should i just get a new one? If its necessary to replace where can i get one?