How old is the wiring in the car? Is it original? If these are both yes, then you could be dealing with old age like cracking. Getting yourself a test light would be first on my list. That way you can test to see where you have juice or not.
As far as dome light go, I had a friend who's dome light was running the battery down. I checked every wire in the loom looking for the problem. finally tracing the wire to the dome light itself and found the wire was dry cracked and had the bare spot exposed in the wire was shorting out against the bare steel roof. After pulling the headliner down to check is when I found the problem.
Finding electrical problems is not hard, just takes lots of time and patience.