Adding an AUX Port to a Car Stereo

Like most people, I can’t stand the music that’s played on my local radio stations. Unfortunately for me, my car’s stereo is an FM/CD unit from the 90’s – and thus has no AUX input. Seeing as I haven’t owned a CD in over 10 years, I had to come up with another solution.

Now, a ‘normal’ person would just go out and buy an aftermarket head-unit with some kind of auxiliary/MP3 input, but I’m a dirt-poor university student with a soldering iron, so I had something else in mind.

The first thing I did was completely tear down the stereo, expecting to have to power it up on the bench and poke around with an oscilloscope probe.


As soon as I had the mainboard out I was greeted with a rather pleasant surprise – the factory had silkscreened the outputs of the RF can, making this mod incredibly simple.


The “L OUT” and “R OUT” traces feed into what I presume are DC blocking capacitors for the amplifier inputs – a perfect place to inject the AUX audio.

So I cut the traces and inserted a 5-Pin 3.5mm Stereo Jack according to this schematic:


This ‘switching’ stereo jack allows the radio to work as it normally would when nothing is plugged into the AUX port.

Modified, the PCB looks like this:


After that I had to find a place to physically mount the jack. The front panel had absolutely no unused space, so I decided to sacrifice the “DISC IN” LED (what a useless thing anyway).


Perfect! Except now the LCD panel PCB won’t fit…


I had no qualms with simply cutting away the section of the PCB that held the light, but that large trace was part of the switch matrix, so I had to run a bypass wire on the back of the PCB.


I temporarily tacked a bypass wire in place, as I wanted to be sure the wire would clear the CD mechanism when reassembled (hence the sketchy soldering).

After reassembling the unit and testing it, everything – including the mod – worked perfectly.

My morning commute just got a little bit more tolerable.