The front passenger side seatbelt on my 1994 S500 Coupe does not retension after the door is closed. In other words: with the door open the tensioner is fine, and once the door is closed the belt is tensiond, but when someone leans forward and then back again, the slack is not pulled back...

I have heard that this is an expensive fix... someone please make my day and tell me I have heard wrong... Could I possibly fix this myself? I have done waterpumps, radiators etc. but the door panel on my old SEC stumped me good...

Am I stuck going to a dealer, or could I trust some independent specialist?

Thanks in advance for the help!!

Steve