I had the squeaky seat issue but it was on my drivers side. I found that if I held the seat belt receptacle out it stopped so my local upholstery place made me two little foam pads trimmed in black to match my interior and I have one either side of the seat belt receptacle.

Almost invisible and works 99.9% of the time. I am pretty anal about repairing my car correctly but this will do, I can't imagine how to try and stop it otherwise.

Matt.