How To: Modify an old car stereo for an aux-in

Hi everyone,

This post is a kind of how to/ how I did it type of post. Today’s challenge was fitting an auxiliary input to my old car which is currently equipped with a fm radio/ cassette player.

I know it would be easier to replace the radio and I know there are gadgets available to broadcast from your phone via FM radio, as well as cassette to 3.5mm jack adapters (there are just too many options for music in the car clearly!). The problem with these methods is sound quality, and in the case of the broad caster vastly reduced battery life, plus I though this mod was quite neat and allows me to keep the standard radio in all of its retro charm (perhaps more relevant/worth doing to a classic car with a period radio to retain stock appearance, also useful if you have steering wheel controls as these can be retained). Plus I like a challenge!


The basic idea is to insert a 3.5mm socket between the radio receiver and the amplifier unit within the radio. By using a switched socket, the radio signal to the amplifier will be stopped when a jack is plugged in and will commence again once it is removed.


Items required for this mod:

  • Switched 3.5mm jack (Maplins: FK20W)
  • Some wire (22-24 gauge or something about that)
  • Soldering Iron and Solder
  • Drill with correct size drill bit to make a hole for mounting the socket (6mm in this case)

Total Cost (to me): £1.99
(Although when removing the trim I found £2, so this mod paid for itself!)


Step 1. Take the radio out of the car, easier said than done in some cases but I won’t cover that here as every car is different and I’m sure a quick google will give you all the help you need with this!

Step 2. Take the radio apart. If you are unsure what you are doing it is best to get some help or find an old radio to practice on first.


Step 3. Locate the rebel base output of the radio/ input to the amplifier. This can be aided by using an audio probe (See here for details). When the audio probe is placed on a pin that provides the amplifier with a music signal you will hear the music you are playing on the audio probe through the car speakers (You need the radio plugged in for this so be careful! Alternatively you can do this out of the car provided you can give the radio a 12V supply and have suitable speakers connected).
This is the only genuinely difficult section of this mod (provided you can solder). I just worked my way along the pins from the radio receiver until I found the left hand and right hand outputs. Once you know which is left and which is right and you are sure, find a suitable place to break this connection with the rest of the board and solder in a feed and return to the circuit board as shown by the diagram below, I found a smaller jumper wire just after the pins connection to the board which was ideal. You can use individual wires or a 4-core wire, typically around 22-24 gauge. Take note of which wires go to which connection.

Radio Diagram.pngtogether1

Step 4. Solder wires to 3.5mm jack. You want the feed to the amplifier to be on the common side of the switch as shown by the diagram, so that the source of the music will be switched from the radio to the aux when the plug is inserted as shown in the diagram below. Follow the diagram for whatever jack you are using to ensure the pins are correct.



Step 5. Ground the switch. You want to ensure that the switch is grounded to the same point as the radio to ensure you don’t get any interference.


Step 6. Mount socket. I drilled a simple 6mm hole in some plastic trim and pushed the socket through from behind and secured with a nut, simples.


Step 7. Enjoy! The radio should automatically switch to aux in when a jack is inserted and you should still be able to control volume and whatnot from the radio itself. A lovely cheap mod to add some extra functionality to your radio!


Additional Information: The above information is correct if the radio you have is stereo (left/ right), but there are some differences if you radio is only mono such as in old AM radios in some classic cars. Some extra jiggery pokery with electrical components is required to ensure that the left and right are merged correctly, if you require more info let me know in the comments below.


*Disclaimer: If you modify your car you do so at your own risk and I accept no responsibility if you damage anything. Just because it worked for me doesn’t mean that it will work for everyone despite my best attempts. If you are unsure please ask for help from a qualified professional.


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