Hacking your car radio – aux input

Nilson writes – “I have a Pioneer DEH1850 car radio system.. it’s Pioneer’s cheapest new model.. no mp3, no blue lcd, no … But, I have an iPAQ rx3715 which I use as, mostly, an mp3 player (though I get some matlab stuff done in it too). So, I figured: why not jack an extra input to the radio and plug in the PDA? So, this is what I did” – Link.