in my car (with MBTex), the squeaks come from the lower cushion rubbing against the lower part of the lumbar support (of the backrest).

what works for me is to treat the rubbing surfaces with a silicone treatment (i use the one for tires, since it's longer-lasting). After some time it dries out and you have to reapply the treatment.

if this is the case with your seat, give it a try ...