Photo heavy post coming up, this is for anyone who wants to fit a rear USB input to this unit instead of having to use the front one. Much of the knowledge for doing this comes from the excellent post by the Russian guy I linked to earlier in this thread.
So first, unit reduced to its constituent parts to get access to the circuit board. You'll need a T9 driver and just go gently and it comes apart easily. There are no ribbon cables that need unplugging to do this.
This is the circuit board we need access to, the 4 solder points circled in red are the back of the USB where the wires need soldering. This unit uses an internal microphone, if you wanted to use an external roof mounted one, the area circled in yellow is where the signal goes in. You would need to unplug the little plug to the right of that and solder two wires, which could then be pinned into the quadlock in the appropriate positions. I'm going to see how it goes with the internal one and may revisit this if that doesn't work well.
Next to solder the wires on. They're very close together so a magnified view and a very fine tip is needed. Not shown here, but after soldering I covered the contacts in a big blob of hot glue and taped the wire to the circuit board to prevent movement.
Now to pass the USB plug through the gap in the metal backplate. This is where I deviated from the Russian post, I preferred to run the cable out the back where it didn't risk being crushed or damaged and would allow an easier run.
Finally I drilled a hole in the back of the case big enough to pass the USB plug through after fitting a grommet to protect it
I will probably try and connect this to the existing USB socket in the glovebox, but for now plugging in a female-female adapter allows me to plug a Lightning cable in and test it.
I have ordered an adapter to do this, and also had to order the correct double Fakra connector for the antenna, which have added another £20 to the price. Even so, that's still coming in at under £200, which for upgrading to a new Carplay/App Connect stereo with a bigger screen still seems like a bit of a bargain to me if you have the basic unit.
So does it work? Certainly does
Here's Carplay on the 330+ using the rear connection.
Parking sensors working fine too
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My 360 camera turned out to use a 6 pin mini DIN which was unexpected, but Google to the rescue showed that it carries both S-Video and composite so I just needed to connect to 2 pins to get an RCA adapter. Not just that, but it turns out that the pin configuration is identical to the old IBM PS/2 plug so I was able to make an adapter out of an old Y splitter cable I had and a phono jack. Unfortunately it didn't work when I tried it but I remembered afterwards that I'll need to code the camera in VCDS, but I'll do that next time.
Then next I'll cut the front speaker cables on the loom and wire them up to a line level converter. The connections to the underseat amp are already in, along with a remote turn on wire (I put them in more than a year ago!) then take the amp to speaker cables, which are currently tucked in behind the headunit, to their final connections.