well, at least on the 126 300 SD (1984), I have replaced the seat switch twice and both times it was the problem.
I haven't seen the motors fail....the seat switch buttons seem to get sticky and fail. I don't know if they can be cleaned or not? Might just try replacing the swtich adn see waht happens. Even at 20 years, my seats seem to move okay. Of course, this car is different than yours.
1) 84 300SD 285K+ Miles (Ver Calif)
2) 84 300SD 175K+ Miles (Ver Federal)