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Not sure how to fix this problem haven't tried but I've owned 3 mustangs and all three have had this issue. Also I think there is another post on here that has the solution
Is the belt running through the plastic guide on the side of the seat near the head-rest? Mine retracts just fine, unless I have it ran through the guide. Then it just sits there.
seems like the 99-04 mustangs have alot of problems with the seat belts not retracting like yours is doing. I have heard of taking it apart and doing something and it working better but can't remember what it is at this time. Your best bet though is to buy a new one or try and grab one from a junk yard