While not as clean as the above install, you can get an FM modulated Aux Imput from Soche which is easier to install. It splices into the antenna wires, and you simply tune your radio to 88.3 and flip a power switch and you have Aux imput.
Amazon.com: Scosche FM-MOD02 Universal Audio...
I have this on both my Jeeps and my daily driver Accord as I didn't want to have to buy new head units. Works fine.
Plan on putting one in the square, will probably put the switch in the ashtray under of over the lighter, and put the aux port somewhere right around the radio.