have a 98 sl500, last night the passanger side seat belt locked up. It will not extend away from the rest position. Any suggestions?

 

I have the same issue but for the driver side !!!

 

on the passneger side there is a switch on the side of the seat for locking the seat belt for a child car seat. maybe try switching the switch. or the seatbelt needs replacement.

 

I have the same problem ocassionly on the drivers side. It seem the pre tensioner gets a grip and the belt will not come out of the back rest.
I simply get out, move the backrest forward and return to normal position. This seems to release the pre tensioner and the belt can again be extracted to put on. A bit of a pain but only happens once in a while.
I do not have an answer on a 'fix'
Good luck