Actually I didn't have to tap into an ACC wire per say, the one included with the factory harness connects nicely through the OS2.
I also found that Hummer provided a capped off VCC and ground wire in the dash behind the stereo. I purchased the base model with the automatic transmission, window tint, side steps and trailer hitch attachment. I did not get the LUX or off-road package so those wires may be for an accessory I do not have.
I'll try to get some good digital pictures of the install on my hummer page in the next day or so. It actually turned out to be a much easier install than I thought it would be.
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