I think you'll find the vacuum pump assembly (that thing you found under the rear seat) doesn't control the rear head rest assemblies in your car. I believe earlier cars used vacuum for the head rests.
My1996 (same year as yours) S500 uses electric motors to raise and lower the rear rests. If you take the upper seat backs out, you'll find the electrical connection for each head rest. You can test them at the connection... but, if they have both failed, its unlikely to be the motors...

__________________
KenP

1996 S500 W140 Coupe