You'll have to take apart the back panel by the back seat. Just look, you'll see the screws and stuff for it and stuff. Once you get that panel off, you'll notice what should be going where. It is hard to type up and explain right now, because it's been a while since I had to fix mine. If you need more help, and have AIM message me at: kumputor nurd
It's a pretty simple job, after I did mine the first time, it only took me 30 minutes to take panel off, fix the belt, and reassemble the car. So, it isn't a BIG job, just takes some time and patience. Be careful doing most of the things as the trim panel may rub against the head liner, which could tear it, or get some silicone sealant from the back window all over it (that stuff doesn't come off to easily
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The belt for the passive belts goes from the track, through a little track-like thing which plugs into the top of the motor. Your belt has probably pushed that off the motor and the belt is probably flying around inside the trim panel. BEFORE you take off the trim panel, you could see if it is in fact that before taking the panel off by opening up the little service door by the edge of the seat (it's a little square that pops out) and turning the knob on the motor. If the seatbelt moves, then it is still in the track properly. If it doesn't move, continue to take the trim panel off.
Once inside the panel, you'll immediately notice the path the belt takes through the motor and into more track.
Sorry I type things kinda out of order, i don't always think in the same sequence I work in
Nick