You must first remove the windshield washer fluid resivour, and the pump is under a plastic panel.
To replace the air ride compressor on a 1999 Cadillac DeVille Concours remove the air shocks located under each wheel and replace it.
Under the window washer tank
I am assuming that you are talking about the air compressor that inflates the air ride. If that is what you are talking about, then it is sitting in the front left up under the hood (where your hood latch is. Sitting in between your battery and passenger side headlight.
Under the windshield washer fluid resivour on the lefthand side.
The compressor will stay running only because there is a leak in the air system somewhere and the compressor is trying to play catch up.It will burn out if left to run continuously ...
round cylinder on front of air ride compresor
I experienced the same problem with my 2001 Navigator and it was fixed by replacing the pump mount (according to the dealer). Fortunately, it was under warranty and didn't cost me anything. Since I had the dealer take care of it, I don't know how involved the job is but it may also be the solution to your problem.
The Level Ride Compressor on your '96 DeVille is located underneath the car near the right rear wheel. Just follow the plastic tube from the air-shock to pinpoint it.
Can't post a picture but, the air suspension compressor relay should be found close to the passenger side front headlight.
The air compressor is located above the rear axle on the passenger side
Passanger side under headlight
it comes on to balance the air ride system according to the weight inside.