For rear seat squeak, there are two areas to check first. Make sure the pivots on the back are lubed, and not too rusty. A little heavy grease helps there. Then check the upper latches. If the rubber pads are worn, you can sometimes get a little reprieve by opening them up and swapping them around or shimming them out a little. You might also just try putting a piece of plastic bag or something over the pin and closing it into the latch. Grease isn't very good here, because it gets on stuff. but if you can make the whole thing tighter, it helps.
Also check if the seat back itself is bent. It's pretty flimsy. If it's bowed in the middle you might want to take it out and stomp on it or something.