I think I may have the same problem. I converted my top to manual operation last summer by loosening the nuts back near the rear seats. Then when I tightened them back down later, I heard motors running, but the top wasn't moving. I didn't worry about it at the time and simply have been using it in the manual fashion. I'll try to get it hooked back up again in the spring.
Manual operation is pretty easy if you just need to put the top up. Loosen the nuts again and pull the top up until it's resting on the headliner. Then get in the car and activate the rocker switch again. The motors should engage that connects the top to the windshield. Do the same thing to drop it (use the motor to release from the windshield , then manually drop the top.)
To get it working correctly again I would suggest going to
www.p-car.com and looking in the DIY section. There are directions on re-aligning the top, replacing cables, etc... If that still doesn't help and you don't get any more help here, try
www.rennlist.com Go to the 993 forum. There are many cab owners and I'm sure somebody will be able to help.
I hope all goes well, I may be hitting you up for advice on mine soon!