This is a really easy (3 Steps) and cheep Under $10.00 DIY fix:

Mini Squeeky Seat repair 101:

If you can roughly locate the source of the squeek run to you local art & craft supply store (Michael's) and get a small roll of 3M doublesided tape and some felt. I would try and get the same color as your plastic seat bolster.

1. Cut the felt into a strip as long as your seat is wide or deep and about 1 inch wide. (With out looking at my seat right now 18 inches x 1 inch)

2. Add the 3M tape to one side of the felt.

3. Push the seat leather back and put the tape side on the plastic surround where your seat is squeeking from make sure the leather will hit the felt it when you let go of the seat or sit in it.

I did all three sides of both front seats. (Left-Front-Right)
Why did I do both you might ask, I am **** and wanted them to match? Nope

I fixed my drivers seat and 3 weeks later, when it got cooler out and the leather is not as soft, the passanger side seat started making the same sound with people riding in the car. ( Why does your car squeek? ) AARrrrrr

The felt will act as nice soft buffer between the plastic and cool "leather" and your squeek will be gone for GOOD.