Having messed with a couple of wiper motors, I think your issue might be easily fixed. That assumes that the sweep to the top of the travel is strong, and the return sweep is as well when you turn the wipers off.
There's a toggle arrangement that switches the direction and it may be stuck, or the linkage inside is detached. Remove the small cover on the side and investigate. If you manually move the motor to the full sweep position, you should see the mechanism switch over. It may just need a little lubrication. That's what that thread the Dan linked to was talking about.
If that doesn't work send it off to
Ficken Wiper Service and he'll rebuild it for you.
