So here is a diagram of the seat circuitry for your car. Since every other function works except the forward/backward slide function, that leaves basically the forward/backward slide motor, the seat switch, and/or the wiring. If you can verify that the wiring under the seat is in good condition without obstructions, chaffed wires, and/or wires pulled too tight, then it's safe to say it's down to just the motor and/or the switch. Sometimes if the passengers front seat has the same switch, you can plug it in to the drivers' side connector and see what works and what doesn't. If the seat works fine with the other switch, then that's all it needs, if not then it will need a motor. If the other seat doesn't have a switch then you can find them at wrecking yards, or just supply power and ground to the motor in question to see if it works. If it does, then you know you kneed a switch and if not then a motor.
I hope this info helps!~subyguru~