I've been checking into this I also have an RTL R/S no navigation. In my old chevy truck I had installed a kenwood head unit and a 10 disc changer. I went to this site
www.installer.com and they had a harness that interupted the changer signal with an auxillary input. When my mp3 player was on I pushed the changer button on my head unit and I would hear the mp3 player play, when the mp3 player was off I had the functions of my changer. It was an easy install too. Basically the harness plugged into the back of the head unit and the changer plugged into the harness and on the harness it had 2 rca inputs that plugged into my 2 into one cable that plugs straight into my mp3 player. I wrote the owner of this business and he says they just installed one in a ridgeline this past week. The part he recommends is
HON03-PC-SON made by P.I.E. electronics. I have yet to order one at this time. I've been checking the forums out lately about this and speaking to the dealership about the aux port. I just purchased my truck this past saturaday and I already miss my hard wired mp3 player, the fm modulation sucks..... Again here is the site for the harness
http://www.installer.com/aux/index.html I'm gonna order one sometime this week and give it a try. If the stock stereo has the aux port and nothing is plugged into it, I know this adapter will work. Thus you shouldn't loose your XM radio feature. I don't have and Ipod, however I do own a creative labs zen xtra 2.0 60gb mp3 player that I've aslo upgraded the harddrive to 100gb with over 14500 songs on it.