The spark plugs on a 2003 Cadillac Seville (assuming the Northstar V8 engine which was the most common engine in that model) are located on
the top of the engine, underneath the plastic engine covers. You'll need to remove these covers to access them. They are arranged in a specific order which you'll need to refer to a repair manual or a diagram specific to your engine for exact placement. A general search for "2003 Cadillac Seville Northstar spark plug location" with images will show you visual aids.
Important Note: Accessing the spark plugs on a Northstar engine can be challenging due to tight clearances. Working on this yourself requires some mechanical aptitude and the right tools. If you're uncomfortable with this type of work, it's best to have a qualified mechanic perform this service.
