Well you had to go and buy a new Accord (I did it too). So we are screwed...

First you have to buy a very specific interface - either HON SNY ITI V.1 from BlitzSafe (www.blitzsafe.com) or SNHOND3 from SoundGate - you can check them both out at www.logjamelectronics.com. Seems that SNHOND3 has
more references and it says explicitly that it can be used as AUX interface
for Honda Accord '03-05 (headunits without navigation only!!); not just for Sony changers!

Go to this web page and read it:

http://www.logjamelectronics.com/sounsnhond3.html

they have compatibility info there and some other helpful links. Good luck!
Let us know if it worked - I read a couple of web testimonials where people said that they did exactly this and everything worked out OK.