Dan,
Shorts are a generic term. The circuit is grounded between the load and the switch allowing the current after the load to find a path to ground before the controlling switch, thus completing the circuit and energizing the bulb. The switches in the door jam complete or close the circuit on the ground side of the load (the lamp bulb). This was a common, as in 2-3 a week when I worked at a Buick dealer.
The only components that are on the circuit that could cause a problem are the headlamp switch and possibly the chime module. Unplug these independently and shut the door to determine if the lamps extinguish. If they do, there's your problem. If not it is then a circuitry problem.
Here is how we would trouble shoot the circuit wiring. Believe me it sounds ridiculous, an old mechanic taught me this when i was an apprentice and didn't believe it until I performed the test, but it works.
Remove all the door switches from the pillars and let dangle by the wire.
Using a circuit tester (fast setting circuit breaker) with a meter, put power to the circuit between the switch and the bulb, NOT the switch and ground.
Since the load side of the switch is grounded (20g black w/white tracer on an 89) causing the dome lamps to stay on, expect an immediate trigger of the breaker and fluctuation of the meter.
The tester breaker will reset, heat up trip and reset. While this is happening trace the route of the wire harness with the meter. The needle will continue to flucuate everytime the breaker trips. No need to remove trim pieces yet as the fields of magnetism will read on the meter if not too thick.
When the needle stops flucuating you are real close to where the wire is grounded, probably cuaght by a moving part & pinched by something.
Repair installation on wire and re-install the fuse.
Good Luck,
Tom