Finally, i've got bored of recording of 6 disks at once, so i've assumed that i need an AUX.
The cassete adapter didn't worked for me, the head unit kept reversing and ejecting in, the same thing with the regular tape, so no chances for that way.
On this forum i've found a cd-changer-butchering method, but i don't want to route such a long cable from the trunk to the cabin, so i've modded it a little.
First of all, i've removed and disassembled the head unit. But if disconnect the wires from the head unit you'll need a radio code to get it working again (any dealer can give that code to you)
The upper cover is pretty easy to remove with a screwdriver.
Then you'll need to detach a tape mechanism, which is held in place by 4 screws.
After that - the front panel is held in place by 4 screws too, so remove it.
Next step is kinda tricky - you'll need to detach a main board from the lower cower - 2 screws holding the PCB, 2 screws holding the rear radiator and 2 metal lips holding the PCB in front.
After the removal flip the PCB over and - here it is, the soldering points of AI-NET connector. All soldering points are marked, you'll need 3 of them, marked as "L", "GND" and "R"
I used a wire from the headphones and soldered it right to the PCB.
Assemble the head unit back and installed it place. I've routed the wire through a J-gate post, since my phone holder (SE Xperia Ray, which i'm using as a player) is installed over there.
Now i've got an AUX-IN, the sound is great even if there's no magazine in a changer.
But what if i'd like to listen a CD? I've downloaded a silent .mp3 track (32 minutes of silence ) and recorded absolutely silent CD, which is now nr.6 in a cd-chager magazine.
So, now if i want to listen a music via AUX i just had to switch to CD nr.6 and hit "RPT" button. If i want to listen a CD - than i just had to switch off the player. Less wires with more functionality.