1. Assess the Problem:
* Type of Sag: Is the mirror itself loose, or is the entire assembly (including the housing) sagging? This helps determine the point of failure.
* Visible Damage: Carefully inspect the mirror's mounting points for any obvious cracks, broken plastic, or loose screws.
* Wiring: Note the location and condition of the wiring harness connected to the mirror. Be extra cautious not to pull or damage these wires.
2. Repair Options:
* Tightening Existing Fasteners (Least Invasive): If the sagging is minor and caused by loose screws, carefully try tightening them. You might need a small screwdriver with the right size and type of head (Phillips, Torx, etc.). However, this is less likely to fix a truly sagging mirror.
* Replacing the Mirror Assembly (Most Likely Solution): This is the most probable fix for a sagging mirror, especially if it's a significant sag. You'll need to purchase a replacement mirror assembly specifically designed for a 2004 Chevy Trailblazer with OnStar. Ensure the replacement is compatible – incorrect parts might not fit or function correctly.
* Repairing the Mounting Mechanism (Advanced and Risky): This is the most difficult option and only recommended if you're comfortable with small-scale repairs and have experience with electronics. It might involve disassembling the mirror housing, identifying the broken component, and attempting to repair or replace it (e.g., a broken plastic clip or a faulty hinge). This is *not* recommended unless you are confident in your abilities; it's easy to damage the OnStar components.
3. Replacement Process (If Necessary):
This is a general guide; the specifics might vary slightly depending on your Trailblazer's trim level. Consult a repair manual or online video tutorials for your specific model year for more precise steps.
* Disconnect the Battery: This is crucial to prevent accidental short circuits while working with the electrical components.
* Remove the Mirror Housing: This usually involves carefully unclipping or unscrewing the mirror assembly from the windshield. There might be clips holding it in place, or screws hidden under decorative covers.
* Disconnect Wiring Harness: Gently disconnect the wiring harness connector from the rearview mirror. Take a picture or make a note of how it connects before disconnecting for easier reassembly.
* Install the New Mirror: Carefully align the new mirror assembly with the mounting points and secure it in place using the clips or screws.
* Reconnect Wiring Harness: Reconnect the wiring harness to the new mirror, ensuring a secure connection.
* Reconnect the Battery: Once everything is properly reassembled and connected, reconnect the battery.
* Test OnStar Functionality: Check if all OnStar features are working correctly.
4. Where to Find Replacement Parts:
* Auto Parts Stores: Local auto parts stores (like AutoZone, Advance Auto Parts, NAPA) can help you find a compatible replacement mirror assembly.
* Online Retailers: Sites like Amazon, eBay, and other online auto parts retailers often carry replacement parts. Double-check compatibility before ordering.
* Junkyards: You might find a used mirror assembly at a junkyard or salvage yard, but be aware that there's a risk of the same problem recurring.
Important Considerations:
* OnStar Functionality: If you're unsure about any step of the process, it's best to consult a professional mechanic, especially because of the integrated OnStar system. Incorrect repair could affect its operation.
* Windshield Damage: Be extremely careful when removing and reinstalling the mirror assembly to avoid scratching or cracking the windshield.
If you're not comfortable with DIY repairs, it's best to take your Trailblazer to a qualified mechanic for professional repair. The cost will likely be higher than DIY, but it ensures the job is done correctly and the OnStar system remains functional.
