When the power seats fails to move forward and backward in a 993 it is most likily either a bad switch or a stripped flexible drive shaft.  This DIY will focus on how to replace the flexible drive shaft. 

(1). Remove the two philips screw that holds the cover the covers the drive motor in front of the seat.

(2). Remove the drive shaft clip from the motor

(3). Slide the plastic wrap along with the flexible drive shaft cable out of the motor drive mechanism

(4). Remove the drive shaft clip that secures it to part of the frame of the seat. 

(5). Pull the plastic sleeve out and the shaft out of the seat drive gear box on either side

(6). Install the new shaft and line it up with the motor drive gear, you may need to tap on the power seat switch to get the square notch to line up while you install the shaft to the motor.  When installing the shaft to the seat's drive gear make sure that it is pluged all the way in, you may even want to turn the shaft with your fingers to make sure it is mounted correctly before attaching it to the motor.