The Mustangs seat backrest have a two peace frame that were spot welded together and they have lots of support on the recliner side but on the inside where they pivot they lacked support. Does the seat lean back to the inside (right) or twist when you push back on the seat? If you can see the seat flex I would bet money on it you have slit the spot welds. This is an easy fix remove the cover as Dan said and then split the frame from the cushion frame, place a 2x4 on the ground then place the backrest frame over the 2x4 on a corner to corner and stand on the seat to flex the frame straight again. Once you have the frame so it will lie flat on the floor we weld the center opening a complete bead not spots to give support also on each side there is a supported area that the recliner bolts to weld this as well both sides for strength. If all else fails post pics!