GMC Troubleshooting Questions? Ask a Mechanic for Answers
Does the DIC still say that you have a door ajar?
OK. A good scan tool will indicate why the courtesy lamps are on, by querying the body controller. This is where I'd be if your truck were in my shop, since this information is so easy to access (as long as you have the right tool), I'd definitely be looking there for information.
One of the doors or the rear gate is indicating open, or the dome lamp switch is on, or there is something wrong with one of those circuits giving a false indication. Narrow it down to the offending circuit, then see what can be done to manipulate this circuit and see if the BCM responds.
I understand that you probably don't have a scan tool capable of doing this. If not, this is the point at which I would advise you to get this professionally diagnosed. If you can't "see" what the BCM "sees" this is a pretty tough diagnosis, short of a lucky guess.