Below are the wires you need to tap into if you are adding an aftermarket remote keyfob lock / unlock system. Also below are some links to an example product option followed by some nerdy stuff.

This is for my 2006 Hummer H3 and should hold true through 2008 models.

The location of the specific Body Control Module that you need to access is behind the passenger front kick panel (right foot).
The connector is the C1 / J1 that is black in color. The higher one of the two.

Door Unlock control
circuit #194
pin A12
color White

Door Lock control
circuit #244
pin A11
color Light Blue

Horn Relay control
circuit #28
pin A40
color Black

Park Lamp Relay control
circuit #301
pin A24
color Brown with white stripe

Power source for new controller:
pin B5
color Orange

Ground for the new controller
I used the crash bar that is up in the dash. It's a ~13mm bolt with raw metal. I used a ring terminal. This is easy to get to if you pop off the upper/right panel. to get it off, there are two push locks hidden behind the glove box door. This also reveals a great place to put the new controller module.

If you "ground" any of these wires, that function occurs. Even if the BCM is still connected. There is no need to "cut" any wires. If you splice in, there is no harm to the BCM.
For the "unlock" function, the first time it actuates, only the driver door unlock. If you hit it again within a few second, all doors unlock. Just like factory. this logic is done over in the driver door, not the new controller.

I only ran into two issues:
1) The controllers that have provision for parking lights and horn almost all have a "+12V" output on those lines. So if you want that, you have to install a relay to "flip it" to a ground signal for the Hummer wiring.
2) The horn signal from my new controller lasted too long. It was like 3 seconds. that would drive me crazy. I fixed it with some electronics know-how: For the relay control line, you install a capacitor inline. The top side (+) of that capacitor then has a resistor that goes to ground. I used a 330uF 16V capacitor and 310 Ohm 1/4 watt resistor. Made of for a nice "blip" of the horn. What I would suggest you do is use a "one shot" or adjustable relay for the horn versus the standard automotive relays.

Amazon product:
or

The blue blinking LED is a nice "Fake alarm" feature. I do not use the trunk feature, but thought it would be a useful "AUX" feature some day

This is the horn relay I suggest using:
Timer Relay Module, DC 12V Adjustable Timer Delay Turn Off Module Timing Relay Time Switch 1~10sec

 

Great info. Thank you 😊.