[jgeorg]

I have the identical year car and the same situation. The pump is staying on because it's trying to level off the car. It has what they call an automatic leveling system. The pump is located in the left front left section of the engine compartment just behind the left headlight. The rear shocks are air shocks. Changes are one or both are leaking and the pump is staying on trying to level off the car but the air just leaks out. That's why you car is sitting low in the back. You can confirm this by having someone put the car on a hoist and remove the rear wheels to inspect the shocks. It'll be pretty evident when you look at the shocks. It's just a matter of replacing the shocks. Once you do this the car will ride better and stay higher in the back. The pump won't run as much.