Please Help Me Out

I have a 03 Trailblazer LS that didn't come with a auto dimming rear view mirror. I traded a Mits Montero in that had one and I deeply miss the mirror(about the only thing I miss on that thing). I have a auto dimming mirror that is supposedly from an 03 gmc truck although I don't have the wiring harness(comp/temp/airbag notfication/auto dimming). The mirror has a 16 pin harness connection just like the TB. Can anyone tell me where the factory mirror's harness plugs up or is it wired directly to the vehicle wiring system?
All of the TB's I have looked at have the OnStar and no Temp in the mirror. I have pulled the A pillar covers off and there is no wiring there. There is a flat plug underneath the dash that is taped up (not the diagnostic port), What is this plug for? i know it's not the trailer wiring, since I found it further up under he dash.
I have though about ordering a harness and temp sensor for the 03 truck, but don't know how I'm gonna find the pt numbers I need (local dealer not too keen on helping without a vin)
Did the 02 TB mirrors have the comp/temp/auto dimming mirror in them? If that is the case, I could use that harness (as long as it is the 16pin)?

The next thing I am wanting is a keyless entry (I should have opted for the LT model, but they didn't have one on the lot). Dealer says they can only have an aftermarket unit installed. I'm wanting a factory keyless entry with possible security. Can anyone help me here also?

Please help me out, and Thanks ahead of time for the support. Sorry for the long post.