I had this problem before with my 87 190E 16v. The problem was simply that there was too much corrosion built up inside of the switch box.

If you pull off all of the switches and remove the little plastic C-shaped clip around the post coming out of the center then you can remove the entire switch box from the door. Its only held in by two screws. Once the box is out you can open it CAREFULLY. There is a bunch of small metal balls and springs inside. Take out all of the pieces being careful to remember where everything came from. Use some metal cleaner on all of the metal "tracks" inside the box, including the metal balls, to get all the corrosion off. Now you just need to carefully reassemble the entire switch box. Use some electrical grease on all of the points where the metal balls contact. Shake the switch box after you put the box back together to make sure all of the pieces inside are in place. If you hear anything shaking around inside you will need to take it apart and try again.

My guess would be that this would fix the problem you are having if you are able to move the seat in all directions except the incline. There is a lot of pieces to keep track of inside the switch box but the good part is that there isn't any electrical boards inside to worry about. If you put it back together and one of the previously working seat movements doesn't work anymore then you just have to take it apart and try again. Its a really easy procedure. I have done this to the switch boxes in the front driver and passenger door of my car and they both work much better than they did when I bought the car. Hope this helps.