Aux-in Hacking an 04 Subaru Radio

Like many of our cars, does not offer any kind of auxiliary input, and aux-in/mp3 adapters tend to run on the not so cheap side of the price scale. Even a replacement head unit was too rich for his blood. So it was time to wire something to the old head unit. On inspecting the radio’s PCB I managed to locate the traces that carry audio from the FM receiver to the stereo’s amplifier.  Most aux input hacks we have seen involve fooling the stereo into thinking some media is inserted, even if interfacing with the audio lines on the PCB. These require that the tape/CD functionality be altered, perhaps permanently. Even worse you may have to shlep around a blank CDR with a bunch of tracks on it! All just to fool the stereo into enabling audio output.
Instead I target the audio lines from the FM stereo, since radio is always enabled when active. Once the audio traces are located they are severed  and bypassed with a 1/8″ stereo plug. This setup allows the FM audio signal to pass through the connector when disconnected, and cuts off any radio audio once your mp3 player is.