As others have suggested, it might well be a broken weld or section of metal seat support under the seat. Several have mentioned that theirs have broken and I've had both my Mystique and SVT end up with the same problem, so it appears that there is definitely a design/manufacturing/materials flaw.
The front seats are very easy to remove and check. Just move the seat fully back on the rails and remove the front bolts, then move it all the way forward to remove the rear bolts. The seat rails are attached to the floor by 6 bolts - you'll need a #50 Torx, IIRC, preferably a socket drive, to remove them and then you disconnect your wiring harness. I've had my Mystique re-welded at the break. Haven't pulled the SVT seat but guess I should get off my ass and do so
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Regards, Alan
