If the seat cables are anything like what BMW has, then the sheath has expanded with age. This leaves the driveshaft inside just far enough away from the actuator or motor to not engage. The fix, as I did on my BMW, was to find a piece of rubber hose with an internal diameter that's the same as the outer diameter of the drive cable sheath. Disconnect the drive cable from one end, cut a small length of the sheath out, put the hose over the cut area, and tighten with hose clamps.

But then again, Mercedes seats might be different.

Hope this helps,
Troy K.
1995 E420