First thing you need to do is learn all about it. That's so you can diagnose it and fix it or tell a shop what to fix. Many shops, knowing that you're over a barrel, take the "replace everything" approach; that's good for them but not so good for your wallet. That's what you need to avoid.
Basically, there's a hydraulic pump, a mechanical height sensor valve in the rear and a couple of nitrogen / hydraulic chambers (like shock absorbers) in the rear.
You don't state the problem so:
- check the fluid level of the hydraulic system. This hydraulic oil is quite expensive, don't dump any old oil into the system (although someone may have by now).
- slide under the back of the car and look up at the chambers - are they leaking hydraulic oil? look at the height adjustment valve - is it leaking? is its linkage intact and connected?
You should know that the M-B station wagons 300TD, 300TE and the 560SEL / 560SEC also have this hydropneumatic rear suspension so it's not unknown here, just costly to have repaired.