Where are the spark plugs located on a 2005 Chevy Trailblazer?

The spark plugs on a 2005 Chevy Trailblazer are located on top of the engine, under the plastic engine covers. To access them, you'll need to remove the plastic engine covers. They're arranged in a V-pattern, following the cylinder layout of the engine (likely a V6). Specific location within that V will depend slightly on the exact engine (4.2L or 5.3L), but they're generally centrally located near the top of the engine block.