* Check the Wiring: This is the most reliable method. Locate the speaker wires going to each speaker. If they are very thin gauge wire (likely 18-20 gauge or thinner), it's a strong indication that a factory amplifier is present. A system without an amp will typically use thicker gauge wire (16 gauge or thicker) because it's carrying the full power signal. This check requires some disassembly to access the wiring behind the door panels or dash.
* Look for the Amplifier Itself: The factory amplifier, if present, is usually located in a few common places: under the driver's seat, under the passenger seat, in the rear cargo area (behind side panels), or even in the center console (less likely). It's a relatively small, rectangular box, sometimes with a Chevy logo. However, finding it doesn't necessarily confirm it is factory - aftermarket amps can be installed in the same areas.
* Examine the Radio's Wiring Harness: If you remove the radio, you might see extra wiring in the harness that goes to a separate amplifier. This is not always straightforward though, as some harnesses look complex even without an amp.
* Listen to the Sound: While not conclusive, a weak or tinny sound from the speakers, especially at higher volumes, might suggest that an amplifier is missing or underpowered. However, this is a subjective test and doesn't rule out other issues like worn speakers.
* Check the Owner's Manual: Though unlikely, your owner's manual *might* mention the presence of a factory amplifier in the audio system specifications.
Important Note: Many 2004 Trailblazers *did not* have factory amplifiers. The presence or absence depends heavily on the trim level and optional audio packages chosen by the original owner. The wiring check is your best bet. If you are unsure about working with car wiring, it is best to consult a professional.
