Apparently, there were issues with your specific radio part, but I can't find an indication of what problems they exhibited.

However, if the user wishes to switch to their aux input wired into the CD input, they can simply press a button hooked up to the Arduino.

The newer technology also uses finer-pitch surface mount parts, making it harder to work with for beginners. I'd rather not have to burn CDs, although that seems like my last resort, and wanted to know if there was a way to get around this. The easiest is to cut the stereo audio traces or wires going to the volume knob, and hook them up to a DPDT toggle switch. Flashy, modern head units can spoil the look of a car’s interior, particularly if the car is a retro, classic, or vintage ride. And a loose Bluetooth speaker in the back better than a4 channel system designed for the vehicle volume….

Connect the AUX audio signal ground to the signal ground of the S-GND power amplifier (Signal GND) in the TDA7386 power amplifier IC. Wiring the auxiliary input into the CD input is simple enough, but getting the stereo to actually switch to that input is harder. I just installed a 2007 STi HU (C-124) with AUX and nothing happens when the iPod is connected I press CD/AUX. Spliced a jack into these, and stuck an old Dension iPod adapter cradle thing in (starts / stops iPod with engine power etc), and stuck a CD with a 74min blank track in the deck.Some big, 2200u and 3300u low esr caps, dc-dc isolating converter and even a low noise regulator had to be used. Alternatively, you might find that there’s a button on the deck that switches the radio into cassette mode whether or not a tape is inserted. Overall, all you need to do to hack in an auxiliary audio input, whether Bluetooth or otherwise, is to get your signal to the amplifier in the place of any other. The 3-Way CD/Aux Stereo Switch is a compact switcher box allows connection of up to three stereo sound sources, to one amplifier.