Review on Metra GMOS OnStar Interface Systems by Scott Rao

Installed directly on a GMC6 Envoy 200 by a friend. However, after installation, the sound quality was terrible. It sounded distorted and hoarse. I checked all my connections and they looked ok. I ended up running a +/- wire from one of the 4 speakers, bypassing the box it comes in. I connected the speaker wire directly to the factory wiring harness and the quality was 10x better and the sound sounded like it should. I ended up having to do this with three other speakers (6 wires). If you have experience installing car stereos, you should be fine. If you're a beginner, hire a pro or ask a friend who has experience.