Where is the starter located on a Cadillac sts?

The starter on a Cadillac STS (depending on the year and engine) is typically located on the bottom of the engine, towards the passenger side (right side when sitting in the driver's seat). It's often difficult to access without some removal of components. You'll likely need to consult a repair manual specific to your year and engine for precise location and removal instructions. The manual will show diagrams and details much better than a general description.