Ok, found it in Rave, and am now even more confused. Rave 86.77.31: Receiver - alarm system - models with sunroof: Shows a picture of an alarm receiver in the same position as mine. (I don't have sunroofs)
The picture for models without sunroofs shows a picture of the dashboard with the radio and HVAC control removed with some large module behind the HVAC control, even though the text talks about removing the rear door upper trim finisher, headrests, D post trims, and rear interior light.

I have checked the wiring diagrams in Rave and can find no mention of this alarm receiver in any section: anti-theft alarm, central door locking, etc.

PS: alarm is functional with confusing wire hanging ungrounded. Key fob and door lock arm and disarm alarm. If door is opened from inside (window down) while alarm engaged, horn sounds and signal lights flash. I apparently do not have superlock enabled in the BCM as locking with fob or key twice within 2 seconds do not prevent door from being opened with interior handle.

 

You have to remember that that ungrounded wire is probably grounded once the headline is in place.
I imagine that the headliner pushes that wire up against the metal roof, there is probably a screw laying around somewhere that used to hold it in place.
Or we can imagine that since you have the base model and since all trucks were wired the same they just left that unhooked since your model did not require it.
Or maybe your truck was built during 2nd shift on a Fri. 30 min before closing time.
Or maybe 1st shift on a Mon a hour after a starting time.
Or by the new guy.
Or the old guy who is about to retire.
Or the disgruntled employee.
Or the college kids working for the summer.

 

Concerning your "Or"s: most likely one them is correct. There have been a few things on this vehicle that have lead me to that conclusion.

I feel quite stupid this morning. I decided to check the Rave wiring diagrams one more time. I went to the same diagram, and still not seeing the alarm receiver, I hit the page down button. Lo and behold, there are four pages of alarm wiring diagrams, and the receiver is on the 3rd page.

From the diagram, the receiver has two wires from the BCM, and TWO ground points. So, I am going to ground the wire, that wasn't previously grounded.

What I am really curious about, is if this extra ground may increase the range of the key fob. Ever since I have owned it, the range of the fob has been much smaller than any other vehicle I've seen. I can't unlock with the fob unless I'm closer than 5' from the vehicle. We will see.

 

LMAO. Confirmed. The random loose wire IS a ground which SHOULD have been grounded from the factory but was not. My key fob now arms and disarms the alarm from 25' away (like every other vehicle with a remote) instead of no more than 5-10' away.

Freaking Friday afternoon and Monday morning builds....

 

That's the reciever unit for your key fob to lock/unlock the door. That ground wire is your antenna. Fancy ain't it?