i had the same prob too. The driver's seat drive cable (bowden cable) just needed to be stripped off it's insulator and fit it back. As for the co-driver's seat, it's the drive cable itself is worn. Both ends of the cable is squared shaped. The end that is shuffed into the motor is worn out thus causing slippage.
This guy that i took it to, called everyone AB, Stuffgart for a replacement cable but non of them have it in stock. Need to be ordered from Germany. So what he did was he laid some strong adhesive bond over the worn end and let it set for a decent 8 hrs. It takes that long for the bond to set.
Once dried, it's as rock hard as metal. All he needed is to shape out a square head over it. JOB DONE! and now it works like new!