Seats stop moving on a Mercedes Benz 420sel when the doors are closed does anyone know why?

The only commonality the right and left seat systems share with the doors is a ground point. Ground point "W12" is located by the center console and goes to the door switches and the seat control switch. The ground for the seat switches is poor and not allowing the seat control unit to work. When you open the door the door switch provides a better path to ground than the W12 ground point. You need to trouble shoot the ground to the switch. They are brown wires.

Hope this helps.