It just so happens I am doing just that.
All I have so far is a plan, but I believe it's sufficient to work.
So first, I have a 2005 year model Grand Prix GTP, which I learned was originally a rental vehicle, so naturally there are no extra options besides the supercharged engine... no OnStar, no uplevel DIC, no TAP Shift, etc. Your vehicle must be 2004 or newer. 2003 and earlier used analog technology, which will not work now, and to the best of my knowledge, 2004+ OnStar equipment is incompatible with 2003 or earlier Grand Prix'.
OnStar helped me tremendously in my 2007 Saturn Ion when this chick t-boned it. You know how it works I'm sure so I won't go into details. Naturally I want it again.
There are several components hidden here and there to make OnStar work. Here's a quick list:
1) Mirror
2) Vehicle Communication Interface Module
3) GPS Antenna
4) Cellular Antenna
5) Sensors in the front doors, front, and rear to detect collisions
You have many obstacles before you. The first of which is this. GM says in the Service Manual:
"Important
The vehicle communication interface module (VCIM) has a specific set of unique numbers that tie the module to each vehicle. These numbers, the 10-digit station identification and the 11-digit electronic serial number, are used by the National Cellular Network and OnStar® to identify the specific vehicle. Because these numbers are tied to the vehicle identification number of the vehicle, you must never exchange these parts with those of another vehicle."
I think they'll let you do it. Maybe with some minor lying.
Now obviously there's the mirror. Grand Prix OnStar Mirrors were the type with three buttons and integrated microphone. Grand Prix never had an option for compass and/or temp in the mirror, so if you want it there, you'll be doing some intricate modifications (beyond what you've got to do already.) If you want compass and temp, look at the how-to's here for how to upgrade your DIC to the uplevel 5-button version, and this will add temp and compass there. Auto-dimming mirrors are another story.. you may find a compatible auto-dimming mirror without compass and temperature that is compatible. There are many many versions of the mirrors.
Next, you'll need the the Vehicle Communication Interface Module (VCIM). This is the core of the OnStar system, containing cellular and GPS electronics. It is located behind the radio under the IP. Three bolt holes are present where you can install it.
You'll need the antenna(s). Antennas of the time had cellular and XM capability in the same package (I think). Not totally sure b/c I opted to get the brand new "sharkfin" antenna. It has GPS, XM, and Cellular antennae all integrated and looks sexy. But if you go with the factory setup, the Grand Prix had a compass module and GPS antenna mounted to the rear package shelf (where the rear speakers are,) and the cellular antenna mounted on the roof almost right above the mirror (prior to '05, comes with complimentary roof leaks,) or above the back seat ('06 onward). You can put it where you like really.
The collision sensors are the same ones used to deploy the airbags I think. So if you have side curtain airbags, you should have the side sensors. Everyone has the front sensor, and I don't know about the rear one.
That's as far as I've gotten.
There are numerous wiring considerations, and schematics will likely be necessary to connect everything. I hope to find a salvage vehicle to tear apart and get the original harnesses, or at least see how it's connected.
I have the GM Service manuals through 2005, so if it changed significantly afterward, I don't know it. One thing I know about Gen 5 OnStar in the Grand Prix is that communications are Class 2 Serial, whereas other vehicles often used GMLAN instead. I wonder if the Grand Prix electronics were ever upgraded to GMLAN communications standards before its discontinuance...
This is the skeletal information. I'd be happy to give you access to my Service Manual (it's online) if you like. It has detailed procedures, pinouts on the component connectors, rudimentary component location diagrams, plus all sorts of other information you might like to know.
