Ok, try this.
Remove the door switches, one at a time and disconnect it. Tape up the wire end so it doesn't short out , tie a string around the wire end and drop it into the threaded hole in the A-piller so you can fish it out after. Close the door and see if the lights come back on. This method will eliminate the door switches. A meter will only show you continuity while pushing the plunger into the switch. You may not be pushing the plunger in deep enough when closing the door or it may only be making intermittent contact.
Second thing is the headlamp switch. It has a setting for turning on the interior lights that may be defective, possibly shorting out somehow.
Cheers!!
