Seat belt won't extend
 

I recently bought a used driver's side seat belt to replace my original that is fully extended and won't retract. The new (used) one has just the opposite problem: it's been fully retracted and I can't get it to extend more than about 5-10 mm before the mechanism locks up. Is there any way to release the mechanism and allow the belt to extend without locking up? Otherwise, I need to try to return this one.

Thanks for any help.

What year & model car?

MOST MB belts require that they are the EXACT position that they are mounted before they will pull out.

Sorry, it's a 92 300E.

Return it, it is possible that it was in a crash and the pyro element has fired.

Will firing the pyro element leave any obvious evidence? Everything looks very clean, without looking like it's been cleaned.

That I don't know, I've not been unfortunate enough to find out.