For the rear-axle only SLS (no adj. ride height), most of the time the "susp. inact." light is due to a faulty self-levelling sensor(s). The light usually comes on when you're parked on s steep incline/decline, then goes away once you start driving on a level surface, but if it stays lit you should get it checked out, or just replace them both like I did. The sensor arms get gummed up with road grime/mud/dirt/water and will eventually seize up or stiffen up.
BTW the sensors are about $100 each, and only available from the dealer.