Since the tailgate wont open, you will have to figure out how to drop the tailgate manually for the next test. Both top latches need to be tripped to open it, -NOT- the bottom drivers side.
Once you get it dropped, the body to tailgate wiring loom can be seen in the lower right corner. Using a pick, pry it out, and unplug it, and check for corrosion, and other nasties.
About the Autel and Tech2, theres a video series I recently posted...
Not sure where to put this, if it needs to be moved, go ahead.
gmtnation.com
..where a owner of a XUV uses a Autel to control some features of the tailgate. The Autel also shows a map of various modules thruout the truck, and if they are communicating.
The Autel units are not cheap.. Anywhere from 1000-4000. Very expensive to be honest. I am still debating if I want to get a Autel or a Tech2 myself. Tech2 is a lot cheaper, from 100 (computer clone version), to 2500 (actual handheld unit with laptop).
One of the big recent problems with the Tech2 actual handheld is they are coming from China, as they are fakes (but still functional) and a few have gotten confiscated coming into America.