Where is the ect located on 2003 Chevy Trailblazer?

The Electronic Control Throttle (ECT) on a 2003 Chevy Trailblazer isn't a single, easily identifiable part like a sensor you can point to. The ECT system is integrated into the throttle body itself. Therefore, the location is on the throttle body, which is situated on the intake manifold of the engine. You'll need to look at the engine bay to locate the throttle body; it's a large butterfly valve assembly connected to the air intake system. The ECT is within that assembly and not something you'd generally need to access directly unless performing a repair or replacement of the throttle body as a whole.