I think it depends on whether or not the car came with a 'Premium' sound system from Toyota.
I may be corrected on this, but I believe that the rear subs were not standard on all MR2's.
If the wiring is already there though it will be a plug and play job.
You'll pick up a rear sub box on here cheaply.
I recommend you upgrade the speakers to more modern ones once you get the sub box.
I used this one from Maplins as a plug and play replacement and the quality of sound is better than OEM.