Pimp Your Car's Stereo. Adding Bluetooth to Your Head-unit. ($2,- Project)

Modern times ask for modern solutions.
Many of us drive cars older than the modern smartphone. And there lies a problem. How you're gonna stream your favorite music to your speakers.
A lot of cars have a AUX (auxiliary) port to connect devices such as your smartphone. But having loose cables in your car and connecting your phone every single time... That's like having a casette-tape with a jackplug sticking out of it.

This solution took me about 3 hours to complete.

Before you begin, please know that a car's stereo is delicate electronics. Be gentle. :) Proceed at your own risk!

The first picture shows the old situation. A jack plug sticking out the front panel. The second shows a solution you can find on Ebay for around $2,-. This is a stick that's powered off the USB-socket of your head unit. (a phone charger also works)
This device makes a bluetooth connection and plays everything you would hear on your phone through the jack cable.
A fine solution, but not clean. Also, you have to change between the bluetooth-device and your USB-stick if you'd like to listen to that source.

There is a better way.

Use some tools to take out the stereo from your ride.
This model is a Sony CDX-GT450U. (about 4-5 years old) I found it really nice to work on. Easy to take apart.

What you want is to measure the pins on the header the detachable front connects to, since that's provided with USB 5V and connects to the aux-socket. The pins are measured on the bottom of the main PCB, so nearly all metal casing parts have to be unscrewed.
Start by taking the upper plate off. From here you can see the space that's unused in this casing. On the right, there is a nice, big gap between the CD-player module and the side of the radio. That is where we lay our focus on.
But first we need some measurements. Take the plastic plate off which has the CD-slot. On this model, it's three screws on the frontside and some clips on the sides.

Now you can take away some more screws. Three on the PCB, two on the far ends of the backplate and two more on the upper rear end of the sides. You end with the main PCB with the CD-drive on top and the backplate still attached. Don't try to remove the backplate, since the amplifier module is attached there somwhat permanently.
With a bit of gentle fiddling, the CD-drive can be removed from a socket on the main PCB. I didn't disconnect the flat cable that's between the drive and the PCB because i was affraid to break it. :)

Flip over what's left of the radio and start measuring.

Take the jack cable that came with the USB bluetooth reciever and put in in the socket on the front panel.

Take any old USB cable (USB A Male on one side) And cut it in half. On the USB cable, we only need the red (positive) and black (negative) wire. Stick this cable in the socket as well. Attach the front to the board.

Take any multimeter and set it for resistance mode. What you do now is you take one of the probes on your multimeter and hold it against one of the USB wires. Now you try to find a place where the resistance from the cable to the connector on the PCB is (near to) zero. Take note of these positions. This is where you solder the cables onto.
You can do the same for the jackcable. Measure between the front contact of the cable and the PCB for example and take note of your measurements.

On this board, Sony was so kind to provide me with a nice, unused place where another header should be. Or maby the same sort of header, but a different model. The point is, this can be used perfectly to connect wires through the hole, as these contacts match the original header.
The dots of solder covering the holes are easily punched through with a sharp tool with a small diameter. I used a pointy voltage detector. Maby the probes on your multimeter are small enough.

Remove the outer case from the stick. Here you see the USB header and the jack socket. I took them both off.
First, connect the stick to a powerbank (or any USB powersource) and make sure you got the polarity correct. The outer pins are the ones used to power the tiny board.

Next, use some desoldering whick (lytze) and a soldering iron to remove as much solder as possible. Then apply some tension on the headers and try to free the contacts using a soldering iron. Be careful not to break the board or destroy the contacts.

You should end up with a tiny piece of PCB. Solder the wires on according to the measurements you did earlyer with the pin header and the cables on the head unit. I snipped the jack cable that came with the stick in two and measured the colours and matched them with my previous measurements. But any colour is correct, as long as you match the contacts of the socket on the stick to those of the head unit.
If you use any other cable, make sure it's rather flexible. The islands on the stick are not built for much force. That's why i put some hotglue on the wires where the jack port used to be and some more to fix the wire and the PCB to the original casing.

The last photo shows a stick with wires dangling out.

Here i found out the CD-drive can be disconnected. From here out, it's really easy soldering.

Cut the wires sticking out of your newly made reciever to the desired length, strip them down and solder them where you thought they should be placed, based on your measurements.

It should look like the second picture.

Put everything back together like you took it apart, leaving the top cover off. Take it for a test in your car. If something is wrong, try to solve it.

Assuming you have followed this instructable correctly, and my directions were clear, you can now connect your phone (or other bluetooth device) and listen to music in your car, wireless!

So take it out one more time, and fix the reciever in place with some hotglue. I placed the unit as close to the plastic front cover as possible, providing the best reception of wireless music.

Admire your work. You've kept full functionality of your head unit and added a bluetooth audio-connection, costing you less than 2 litres of fuel. No more sloppy cables dangling around.

This instructable can be used on nearly every car stereo that has both a AUX-in and a 5V out. To power the stick, you can use a phone charger as well.

I doubt if there is a cheaper way to get the same result.

Happy soldering! :D