It's not the most fun thing to do, but you do get quite a bit of access to the belt mechanisms if you remove the boot cover, remove the four bolts that hold in place the top seatback panel, remove the top and bottom seatback panels and, finally, remove the lined aluminum panel (about 10 bolts on that one as it is connected to the prop shaft cover).
It's extreme, but if nothing else has worked, it might give you a better picture of what is going on.
