Plug-n-play up to an 05, maybe 06.
I can't remember off hand if the Harmon Kardon radio debuted in 06 or 07. My 07 has the HK.
If you're willing, and able to, the wiring connector to the radio on your bike can be repinned to work with the HK radio. That would open you up to radios to the 13 model year.

 

i thought the stereo in an 04 was the radio sound unit that had a cassette tape player? from what i've read, you can do p & p on the hk unit on a radio sound unit, but, you are going to need to change speakers as well, since the rs are 4 ohm and the hk is 2 ohm.

 

You can make a newer unit work. Changed out the unit on my 2001 Ultra (cassette) with a Harmon Kardon from a 2009 Street Glide, however, I did remove the Tour-Pak and only running front speakers. The unit on my 2003 is a CD unit same as yours and I don’t recall the terminal configuration. So, pull your outer fairing and have a look at the connections. You will need to replace your existing speakers due to reduced ohms on the newer unit. But yes … it can be done.