auxmod interfaces with the unused tape/MD addon interface port at the bottom of the head unit, that's why people must use the tape/MD button for the auxmod, because it is behaving as if it is a tape player
the CD button DOES work even if you have a single disc player to cycle over to the external CD changer mode... the 04-05 head units are actually completely modular... base headunit the same for most 3s, it's the CD module/drive that's different depending on CD changer or single disc