The 5 button DIC with temp and compass is very easy to do.
All you need is the front temp sensor that goes behind the bumper/grill,
the compass sensor that goes under the rug under the back window, and the 5 button dic with the matching trim.
The best place to get this stuff is from a junkyard locally as most online sellers realize the demand and the price is usually 200+ dollars.
If you cant get the temp sensor off the wrecked car due to being smashed to badly the dealer has em for about 10 bucks. The car has all the wiring already regardless of what trim it is.
I know of one person who couldnt find the connector in the back for the compass module but im guessing it should be there, To look for it you can reach your hand in from in the trunk where the audio amp would have been between the two speakers and feel around for the unused connector, Its around the left speaker.
On the onstar topic did you manage to find that second white connector?