I have an ongoing problem with my '91 Buick Skylark. It keeps blowing the interior light's fuse. I've taken the car to my mechanic four times now, and he can't find the problem. He says it is intermittent, and he can't make the fuse blow while he's working on it.
Any problem being intermittent always makes the job that much tougher. Rule number one, when the courtesy light fuse blows, look inside your cigarette lighter receptacle. You would be amazed how often a penny can find its way in there, and poof, there goes the fuse.
After that, it's in your mechanic's hands. However, you may still be able to help by telling him exactly what you were doing when the lights went out. Examples: closing the door, going over bumps, opening the center console or using the cigarette lighter.
Often times an intermittent short is caused by electrical wire insulation that has worn through over time, usually by rubbing against a fairly sharp metal surface. When the hot wire touches metal and the current finds a path to ground, the fuse blows. That may happen intermittently under certain conditions. Vibrations, bumps or other movement in the area of the short can cause the fuse to blow. This is why telling your mechanic that the lights went out when you opened the center console door can help. He can go straight for that area to see whether the circuit shorted to ground in the wiring harness just below the console.
Other than that, you need a qualified technician with a wiring schematic to analyze the circuits and start digging. Of course, some mechanics are better than others when it comes to automotive electrical diagnosis.