1. Service manual, Group 8--Power Seats. Get
very familiar with the Electrical Tests described therein, which mainly focus on the connector to the switch assembly under the seat.
2. Since your seat works in other directions, you already know you have power to the switch, so the tests for power to that switch from under the dash aren't necessary.
3. In my experience, the tracks and motors/drive mechanism are built like a tank. The switch is the weakest link.
4. First test is to observe the dome light when you try to make the seat go back and forth. If it dims considerably more than in other directions that do work, that indicates that you may have a track jam or a bad motor/drive assembly.
5. If not, unplug connector from switch and get it out in the open so you can do the tests. Remember that the switch is always "
hot", meaning it has power to it even when your ignition key is in your pocket, so be careful about making sparks somehow. Make the jumper-wire tests at the connector to see if your seat moves. If so, it's the switch.....which is very common.
The switch
can be rebuilt if you're handy and it's not toast inside or you can deal with this guy, which is highly recommended:
Devinism | eBay Stores
Before buying a new one ($$$$) or attempting to rebuild it, you might try HEAVILY blasting the internals of the switch with contact cleaner. Blast that thing! Blast it! But please protect your eyes in the process, as the cleaner coming off the switch can squirt in any direction and it prefers unprotected eyeballs.