Introduction: $1 AUX Stereo Input Hack

Here’s how to hack into your Honda Accord’s radio and add an
auxiliary input.

Two methods are presented here, the first hacks into the
radio, which can be applied to all head units. The second method hacks into the external Honda accessory port, but requires that you have any Honda accessory attached to enable that port.

Step 1: Disassemble the Radio

The first step is to remove the radio from the vehicle.

Next, the side brackets need to be removed to get the CD/Radio player off of the radio face.

Once that’s out, you’ll need to open up the CD/Radio player, since we need to tap into its motherboard to get an auxiliary line.

There’s at least 14 Philips screws that hold on the heat sinks. Remove all screws hold the case and slowly take them apart. Take note – there are some longer screws that have to back in the same location.

Once the case is opened, remove the four screws that hold the CD player to the motherboard.

Step 2: Motherboard Overview

Once the motherboard is removed from the case, we can analyze its circuitry. Everything is organized into sections of the radio, depending on its function. Towards the top and right are the inputs, including aux, radio and CD input. They all head over to the sound processing unit, situated centrally on the board.

The inputs from the buttons and LCD on the radio face also interface directly to the SPU. The SPU is responsible for selecting the input, applying the corrective filter, driving the LCD display, equalizing the sound, and outputting it at line level to the transistor amplifiers to the right of the board. After the sound has been amplified, it heads out to the speaker output plug on the back of the radio to the speakers.

Step 3: Analyzing Radio Inputs

Here’s what the back of the 14 pin accessory plug looks like, with its pinouts. It uses the I2C protocol to communicate with the SPU (Honda calls it M-Bus), and allow it to select this as an input. That’s why in order to use the second method in hacking in an aux line, you actually need a device that can speak I2C language.

Now the next logical thing I tried tapping into was the CD player. The connections for an analog output from the CD player to the SPU do exist on the board. However, I traced the circuits and they end up nowhere on the board. Upon further reading, it seems like Honda uses a digital output for the CD changer, which is not compatible with the analog signal coming out of your MP3 player or phone.

The board also has pinouts for a built in separate aux input. It works similar to the Honda Civic radios – two wires get shorted together for jack detection, putting it into aux mode. Then the left, right and ground are your analog inputs. However, I traced the inputs and they too lead to nowhere on the board. It seems that the spot for its filters (capacitors and resistors) before it goes to the SPU were mapped on the board, but never soldered in, making this an unviable option.

Next we move onto the radio modulator. This is underneath the big shiny rectangular box on the board. Through some experimentation, I found that I can inject an analog mono signal and get it to override the FM signal. To do this, you need to solder wires from your Aux cable to the AF OUT and ground of the board.

Step 4: Soldering the 3.5mm Audio Cable

Here’s the heart of the project, a $1 microphone extension cable from the dollar store. Use a 3.5mm pinout to determine which wires are left, right and ground.

I cut and stripped the wires and soldered them to the radio modulator. This method isn’t the most legit way to do this, since you are overriding an FM signal which consists of different frequencies in different phases with a mono signal.

Next, I made a hole in the bottom of the case to route the aux wire out.

Time to replace the motherboard in the case, routing the wire through.

Step 5: Reassembling the Radio

The CD player was then reinstalled, along with associated screws.

The case was then closed up.

Take note of these two longer screws on the back of the radio, they’re different lengths than the rest of the screws.

Once all the heat sinks are reinstalled and the case closed up, its time to reinstall it into the radio face.

The brackets are reinstalled on the side of the radio.

This is what you should be left with, a stock radio with an aux wire coming out, and no extra screws.

Time to reinstall the radio into the dash. To use the FM aux
with a phone with jack detection (such as my Samsung Galaxy S5) you’ll need to play the music first on the highest volume, then turn on the radio so it overrides the FM signal.

Step 6: Second Method

The next method is much simplier and doesn’t require you to open the radio. However it does require you to have a Honda accessory which plugs into that 14 pin connector. In my case, I have a 6 CD changer (it’s broken, but that doesn’t matter).

The CD player will be just enough to fool the radio into thinking there’s something at the port to read from. Then we’ll inject our own analog signal into the line.

Step 7: Wiring to the Accessory Plug

Here’s the pinout for Honda’s 14 pin accessory wire.

The best part is this will use the other half of the aux wire that you chopped off for the FM hack, so both methods add up to a $1 hack!

The connections were made by depinning the connector, and twisting the aux wires around, and reinstalling them. No need for soldering!

Here’s what your hacked 14 pin cable should look like with an AUX wire in line.

Step 8: Reinstall the Radio and CD Changer Into the Dash

Install the Honda accessory cable into the radio and route the wires.

I made a hole in the console box and ran my aux wires out.

Finally, I reconnected the CD player and reinstalled it, routing the wires appropriately.

Replace all interior pieces and your done.

Step 9: Conclusion

Now you’ve got two methods to hack an AUX line into your Honda Accord’s radio! This might work for other cars too, since the principals are the same.

I found that the FM aux line, while mono, gives more bass and volume. The CD changer method is much clearer, is stereo sound, but is at line level so you’ll have to crank up the volume to at least 25 to get a decent volume for driving.

Finally you can enjoy your hacked radio that you’ve only spent $1 on, and toss that lousy FM transmitter.