It depends on which air filter you are refering to. Since the engine air filter is pretty straight-forward I'll assume you're talking about the cabin air filter. I have a 2005 and assume it's the same for your 2003 so try looking behind the glove compartment. You should be able to get behind it by opening it and gently bending the plastic part that catches when it is all the way open so that it passes the catch. It will then swing the rest of the way down (you may want to remove the contents first) so that you can access the door to the filter.