There should be a blue vacuum hose underneath each seat.
If you have a vacuum pump to check the system, you can start by checking the complete system in the seats to see whether the lock and button in the seat itself function well or not...
If you want me to, I can mail you some pictures of this system in my 300CE conv. (currently there is no interior in the car and most of the systems are visible).
with a minor 600+k