Let me tell you what I know about parasitic drains and the 2005 Colorado because I spent about 2 months and a lot of research finding mine which was in the end self inflicted,
First The GM spec for parasitic drain is 20 ma. Meaning that after everything is shut down the vehicle must draw less than 20 ma or .020 amps and must be capable of starting unassisted after sitting for 30 days.
To accomplish this the BCM goes to sleep after you have turned everything off and exit the truck. How does the BCM know you exited the truck, that is by monitoring every thing it supplies power to. It specifically watches the interior light drain. So usually after about 1 to 2 minutes of no drain the BCM goes to sleep. If it does not go to sleep it draws around 600 ma. plus what ever is still on. Today mine draws 18 ma. or .018 amps.
In my case I installed an aftermarket mirror with compass and temperature read out. I got the 12 volts to run it from the map lights since it was conveniently near by overhead. Well the BCM watches the map light circuit and saw the small drain from the mirror. It made the decision that I was still in the truck and never went to sleep. It would run down the battery in 2 to 3 days. The solution was to get power from some place not monitored or controlled by the BCM.
I would start by looking at that non-OEM wiring you have under the hood and see if you can get rid of it. After that it is time to get out an amp meter and stick it in series with your ground cable. Start looking for leaks by pulling one fuse at a time to see what affects your drain.
Good Luck.
