2004 Cadillac Deville rear height

The ride height should adjust automatically when the key is turned on. Do you hear the compressor turn on and pumping when the key is turned on? If the compressor is running non stop and the height doesn't change then most likely there are leaks in the shocks. The best way to tell is to get a spray bottle with a soapy solution and spray all around the bag part of the shock while the compressor is running to see if the shocks are leaking. If the compressor never comes on then you will need to begin chasing the issue if it is electrical. There is a kit available to just convert the shocks to a conventional gas filled shock design and a plug that goes into the connector so you don't constantly have an air ride suspension warning message on your display. The kit is fairly inexpensive. The replacement air shocks can be pricey.