My driver-side orthopedic seat adjustments have stopped working. The car is a 1990 500SL with 30k miles up. The passenger side still works fine. Turning the two little dials beside the driver's seat has no effect at the moment. I suspect the air supply has been cut off in some way. Where should I look first?

Photo attached to show seat model, as there were choices at the time.

Momo

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1990 500SL 65k km - until May 11 2004
2004 E320 4-Matic wagon
2004 CLK500 Cabrio from May 11 2004