Hi chocolatemom. Welcome to the forum. Suspect the switch assembly. Since the seat moves forward, you know the motor is ok. There are only three things that can keep the seat from moving in only one direction. Burned or pitted contacts in the switch is the most common. Each switch has four sets of contacts; two for each direction. On newer vehicles, a relatively trouble-prone Body Computer and / or a Memory Seat Module controls the seat motors. They use a different type of switch with only two contacts, one for each direction, and it is still possible to have one that is pitted, but it's more likely the computer itself is at fault. The least of the three possibilities is a defective memory seat sensor causing the computer to think the seat has reached the end of its travel so it only allows it to go the other way. It would be more likely a linkage to the sensor is disconnected.
A trick a mechanic will use is to switch the position of the two wires in the motor connector so it runs in the opposite direction. That will prove nothing is binding in the track assembly.