Does it have a CD changer in the trunk? Is so check the power connection to the CD changer. Quite often the connector gets 'whacked' by something in the trunk. Also,
Preform this test:
1) Disconnect battery ground.
2) Turn the light switch to the 'park' position.
3) Now measure the resistance between the positive and negative battery terminals. (be sure and leave the battery disconnected)
What did you get?
Now turn the light switch to off and measure again. Any change?