I agree with leathermang...my first guess would be that you have a leak somewhere. Is it a "musty" smell? You rear window, windshield, heater core, maybe even the sunroof?....I have a leak in the trunk, and it smells pretty mildewy back there most of the time. Luckily it's not in the interior.
Or maybe you have some rotting french fries in the no-man's-land between the seats and the console! hahaha...Have you ever dropped a pen down in there? Impossible to get out!
Mike