Yeah, I wasn't suggesting that you'd want to make the OnStar work. For starters, it would be useless as our generation of GPs had analog OnStar which literally no longer works since they went digital signal several years back. You'd have to rip out an OnStar receiver from a newer GM. Secondly, even if it worked, it would be a serious pain in the ass to wire everything in for it. The OnStar mirrors have something like a 20+ pin harness. But even if you wanted to just use the self dimming and compass, you'd only have 2 wires going into that 20+ pin harness, not to mention it's a very big, bulky mirror which looks ugly to me. In fact, if I wouldn't have a couple dozen wires to hide or get rid of, I'd switch to a regular Gentex myself since the original OnStar is useless and obsolete now.
The newer mirrors use a 7 pin harness (Gentex started using them several years before Donnelly), but since you already have a self-dimming mirror, it would be VERY easy to just splice it into your pre-existing wires for your current mirror. You already have everything you need; 1 wire running to a 12V+, 1 wire running to a - ground. Otherwise, to go with a 3-pin Gentex mirror, you'd have to go with an early - mid '90s style mirror which won't look right with the newer interiors.