check to make sure you dash light dimmer is not set all the way to max on where the dome light is turned on.
does it turn off with override switch, the switch that makes it not come on when door opens?
if its off and off, and still is on, there is a short to power giving the bulb power when it shouldnt have it.
you can figure out what circuit its getting powered through fairly easily, although fixing it may be much more diffucult.
simply disconnect the negative battery cable from the battery, take a 12 volt DC test light, hook one end to neg battery terminal and the other to cable. if power is flowing light will be on, remove fuses one at a time until light goes off (replacing them as you go).
if the dimmer doesnt have it on, and the override overrides and shuts it off, then a door swtich is the problem.