It is very simple. With the switch on the overhead in the center position, the BCM applies a ground to turn the dome and trunk lights on and off. With the switch all the way to one direction (I think right) NO lights EVER come on. WIth it all the way to the other direction (I think left) they come on and stay on. So, center=automatic.

If lights won't come on in the center position when you open the trunk or either door, I'd blame the three position switch OR the BCM. The door switches wire to the central locking pump/security module and the clpsm uses the CAN BUS to tell the BCM that a door is open. So it COULD be a security system issue.

If you are trying to isolate your problem without the wiring diagrams in the service manual, I don't think you will get very far. I'd download the service manual (from this site) before we went any farther.

My money is ALMOST on the switch in the overhead......