On a 1998 Cadillac DeVille with air ride suspension, the air compressor (pump) is typically located
in the right rear of the trunk, near the spare tire well. However, it's not always in the exact same spot, and some slight variations might exist depending on specific model year and options.
It's a good idea to visually inspect that area first. The pump itself is usually fairly large and noticeable. If you can't locate it easily, consulting a repair manual specific to your 1998 DeVille will give you the precise location and potentially a diagram.
