Sorry for the sluggish reply....
The service manual lists its location as 'behind dash panel, center', but doesn't show a picture. I've never personally put my hands on one out of an '03, but my guess is that it'll be in the dash near the blower motor. You'll probably need to remove the stereo and trim from the center stack to access it.
Typically they're placed in the plenum somewhere near the blower motor so the air passing through the system can cool the resistors as they tend to get hot while the system is running. That may or may not be the case for yours. Once you have the new part in-hand, it shouldn't be too difficult to track down. You should be able to find it at just about any auto parts store, or
www.fastpartsnetwork.com
The other possibility is that your switch went bad. The switch provides the ground path for the motor through the resistor pack. If the switch was bad, you'd have an open circuit in positions 1 and 2, and a closed circuit in positions 3 and 4. If you're looking at the back of the switch, you'll want to check for continuity between pin 15 and the four other pins. If the circuit is complete between pins 9,10,11 or 12 and pin 15, then you know the switch is good.
Hope that helps! Let me know if I can be any more help!