The harness may or may not be there. After hours of searching, my car does not have a compass wiring harness, in spite of having an ambient temp harness and OnStar harness.
You can still put in an auto dim though, the only required wire is the 12v power wire, it all works with just the 12v wire. Find the pin out online and just tap into and route a power wire from another point in the car. I tapped into my sunroof power wire.
If you have OnStar stock but no auto dim, like my car, it still isn't plug and play, it's just a tad more convenient to route the power wire into an existing plug.
