Welcome to Mustangworks!
We had a similar problem on our 2002 Coupe. The passenger side belt got twisted in the retractor and it locked up. Luckily, it was still under warranty, they had to replace the entire seat belt assembly because they couldn't unjam it.
Your 99 is probably out of warranty, but check with your Ford dealer to see if there has been any recall on that, if so possibly it can be repaired under warranty anyway. If not, there are plenty of donors lurking in the salvage yards.
Check with
www.stangparts.com. They specialize in parting out wrecked Mustangs. Good source for body and interior parts as well as engines/ trannys.