If I were to swap a AC/radio unit with a duel climate button, into a 03 without the dual feature. Would everything work fine except the dual climate feature. Thanks.

 

Yes, provided you have all the trim, some of the early 03 radios don't fit the trim of later 03/04 models.

 

And the difference would be.

 

I don't know the exact difference, and don't want to try and find the posts. I just know that a few people have picked up '04 (or late '03) radios for their early '03s and the headunit doesn't fit the same trim. I don't know the exact cutoff for the 03's either, but if you have a "doggy door" on your trunk, I'd say it's a good chance that your trim won't house an '04 radio.