Welcome to AFM! I would think any of the 94-04 ragtop rear belts would be interchangeable but I don't know if they made any revisions throughout those years. Could be a fairly safe bet. Have you tried fiddling with it to see if it just retracted too far and got stuck?
Sometimes a coin get caught up in the receiver and when the tab goes in later jams up. If never had it happen to me, but a friend's dad got stuck in his corvette until he called someone on his cell phone to come cut him out of it. Always a good idea to have a knife in the console should that happen to you.
Sounds like some real 5 star service you received from the dealer, "So yeah, we sold you some broken crap, good luck figuring it out bro...". And I sincerely hope that it wasn't a Ford dealer who told you that, as they could have found the answer in about 3 seconds.
Anyway, as was mentioned above, a seatbelt mechanism from any 94-04 convertible should work. The only major possible difference between model years I can think of might be changes to the buckle, so maybe keep an eye out for that.
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