You could probably get most of it working if you really wanted to. Like you said, the gray plug would be the most difficult because it is only on the newer trucks. That plug is the BCM controls, so I don't know that it is as easy as just tapping wires, but it could just be for backlighting, ignition position sensing, etc.
Good luck if you decide to tackle it. It's difficult enough on trucks that were designed for the kit. Personally, I'd just find a different aftermarket kit and find a clean way to wire that up.