So my wife and I, being poor grad students, finally saved up enough for a used travel trailer. We love it, but the radio with a CD player is useless to us. So I decided to add an aux cord. I figured a lot of people have this same radio in their trailers so here is what I did.
On the top board there is a spot where the audio is fed out to the speakers (see pictures below) and this is where I added the three wires I stripped from an auxiliary cable. R=right, L=left, G=ground.
Then I notched the cd cover a little to pull the cord through.
The radio (AM/FM) still works and so does the CD player. When a phone or mp3 player is plugged in, the audio overrides the weaker output from the am/fm and cd and has no interference. The radio can be left in either mode and there is no issue.
Unfortunately, I couldn't figure out how to add the cord before the volume controls, so the volume knob does nothing. The volume is controlled by the device only (along with equalizer, balance, etc.) I could probably figure out how to add the input through the CD audio and then play a silent CD but I decided against it. Either way, it works well and was free!
Moderator edit to re-size pictures to forum limit of 640px maximum width.