AUX, it's not all that complicated. - Toyota RAV4, 2.0L, 2004

Actually, the head unit of this car is not ice at all, and does not read mp3, and it is not reasonable to record ordinary audio discs for the sake of 3-7 songs you like. It was decided to make the so-called AUX line input. Having climbed the network, Xcarlink devices were found, but the price for them bite, even on the Ibei the price started on average from 2.8 rubles or more.
Here on the forum of the "raf4" club, information was found on the Toyota AVC protocol. In particular, the posts of Mr. Sonic_ru. Then I went to an excellent site, where actually everything is written very clearly about the protocol, and ready-made circuits, firmware for the microprocessor.
Here is the actual site www.softservice.com.pl/corolla/avc/
Briefly the principle. You can connect a CD changer to the GU. The device is connected to the changer connectors on the GU (+ - TX) and emulates the presence of the changer, after the GU sees that there is a changer (in this case, blende) it activates the line inputs.
And work began to boil. it took a couple of days only to search for components, it turns out in Chelyabinsk it is very bad with spare parts, in particular with microprocessors. Found only one! the processor and then in a small case (seen in the photo). I had to buy a breadboard to assemble the circuit.
Schemes, etc. I will not attach, you can look at everything on the site.
After connecting all this, pressing the CD button on the displays lights up CD 1-1, but there was a minus. This is the difference in sound level between the internal sound sources of the Gu and the player connected to the AUX. The problem was solved simply, a preamplifier circuit was assembled on an operational amplifier (OA) TL72 (TL62).
At the moment, I dismantled the board to transfer the 12-5V power supply to it and assemble the case (I hope AndreyS80 will provide the material).
In the future, I will probably screw the USB stick to the device. This has already been done again by Mr. Sonic_ru based on the FM Transmitter.
If you have any questions, ask.
I spent this on everything, about 500r. The most expensive detail is this model.

Probably they will ask the question: "Isn't it easier to buy a" normal "GU?
I will answer: "Yes, it is simpler, but for me it is not advisable, because I am slowly collecting KarPC for rafik, 70% of the parts are already available"