If you're power steering "squeaks" while holding full lock then it's most likely caused by air in the system causing the pump to
"squeak"... To air out the system:
Let the car idle (below 1000 RPM)
turn the steering wheel to full lock one way
hold for 2-3 seconds
turn the steering wheel to full lock the other way
hold for 2-3 seconds
No more squeaking on full lock, I did mine this morning, took 0.5 seconds (well, 6 seconds but anyway!) and it worked :)
The info's from the Mk2 repair manual, but I guess it will be the same for all MR2's with power steering :)