I have a 1996 S320 that the rear seat head rests do not go down by the control switch. I removed the rear bottom of the seat and found a large control valve assembly having at least 12 electrical and vacuum lines. According to a search I thought this is where the control system for the rear head rest was. This vacuum system appears to do a lot of different things. No noise or anything coming from this unit. What do I do next?

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...

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KenP

1996 S500 W140 Coupe

Thanks for the input but this model has manual raising of the head rests and automatic lowering by the control switch.

Dave,
Sorry, thought they'd be the same because of body (W140) and year. I had strange noises coming from my rear rests and was told they were vacuum powered... when I took them apart, they were electrically driven instead.
Good luck finding your problem. Maybe someone else here knows the answer.

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KenP

1996 S500 W140 Coupe