My daughter's 940 wagon had the same problem. I took it to a mechanic friend of mine and he showed me how to fix it. When the same problem popped up 3 years later I did it myself. But that was probably 2-3 years ago....
Here's what I remember:
take the seat out
turn it upside down
remove the cable from the drive motor.
This part is a little fuzzy...What happens is that the cable covering stretches and doesnt allow the actual cable to engage in the gear drive. You'll have to remove about 1/4" inch of the shrouding from the end of the cable. My mechanic used a bic lighter, I used an exacto knife. Stick the cable back in the drive and it should work. The key to all of this is that the problem is caused by not enough of the metal cable engaging the drive gears. It's really an easy fix- you can do it.
Phil C