It has been done! 

hmmmmm.... Game on Wayne!

I'm going to have to give this some real thought - having a Pi pass music to my head unit and also push data to the screen and receive steering wheel controls is just perfect. I need to make it happen.

To read the steering wheel buttons, you'll want to get the RPi reading the car's CAN Bus!

Thanks for the heads up bud. I thought it may be involved somewhere but I have no idea where to start with that yet, ha ha.

Using the CAN Bus is it possible to push text to the  cd player display? If so that would be incredible.

Glad this is just what you need - I have more motivation to work on it ;-) Right now working on a driver that'd use PCF8574 boards typically used as I2C backpacks for those LCDs, but not in a conventional way - I'll use those boards as button boards which will allow buttons for control of my system to be connected through I2C, just like people typically connect displays. It also uses interrupts, which makes for less polling of buttons - I just need to poll one pin to know whether any buttons are pressed =)

What do you use to connect those displays to your Raspberries? I can write and test a GPIO driver for you tomorrow. I already have drivers for GPIO buttons, PiFaceCAD, USB numpads/keyboards, as well as some MCP23008 expanders and those PCF8574 boards I mentioned above =) Also, I'll get an install script ready, though installation of this is just a couple of apt-gets and pip installs away.

That sounds tremendous - definitely keep working on it! I'm intrigued how you out together those button controls.

I currently use my screens with Arduinos mostly buddy; they are acting as a display for my aquarium controller, displaying temp/pH for a tank. The arduinos also post that information to the Pi via i2c to be displayed on the web interface :-)

Oh, I also have a serial driver - my first LCD received data through RPi->USB-UART->Pro Mini->LCD setup =) I can throw some code together so that it works again, test it and even include buttons if you need them =)

you, sir, are my hero! :-)

I just feel obliged to offer my system because, honestly, it solved my problems, so it might as well solve somebody else's =)

Thanks for your reply mate! You are right - I just need to get stuck in and get playing around with it!

Now I've put a bit more thought into it this is what my ideal setup would be:

A pi, able to stream music from a phone via Bluetooth/WiFi (with the Pi set as an AP), with music also playable locally, via USB.

While eventually I would like to integrate steering controls (some of those pins on the CD changer port will hold the answer to that) I think an interface would be great. What do you have in mind? Oh the intrigue! :-)

Heh, I just looked at your projects page. It makes me think you might be even more interested in it. It's here: https://hackaday.io/project/10001-pylcs-linux-control-system and here: https://hackaday.io/project/7281-linux-control-system . I believe it's a way to revolutionize the way we interact with our Raspberries.

I plan to release a 1.0 version in a week or so - my main focus now is making install/configuration scripts, tidying it up a little and making some neat videos so that I can show people what my idea is and how it's useful. Do you have some HD44780 screens? If you do (and you most likely do), I can tell you how to try this thing out so you can better imagine the potential of it. The thing is Python, but you seem to be familiar. https://github.com/crimier/WCS - here's the V1 code, there's a V2 code in window_manager branch but that's long time from now.

OMG, you have so many projects that are similar to my ideas, I even made a portable Pi-powered music player, doubling as a portable desktop. It also was the same shape as your portable Pi programmer... And I have a portable Raspberry Pi EEPROM/AVR/PIC programmer which I'll integrate in my system soon =D

Oh, and I think I more or less have the knowledge about what you need.
For WiFi playback, there's AirPlay, it's supported by pulseaudio and is more or less the most popular way of streaming music to speakers among DIY crowd, AFAIK. Bluetooth has all kinds of services you can use for this. You'll need some knowledge about what DBus is though, as that's what is used for many things Bluetooth and Pulseaudio =)
USB is easy, there are all kinds of automount scripts. Heck, even I have one which I can test for you =)
I'm curious about the music player for this approach. Aplay is too simple, mocp doesn't have a proper external interface, mpd or even Mopidy could be it but I'm not sure...

Jackpot - those interfaces look amazing! I have a few screens in my office at work, so will definitely look to giving these a go :-)

I agree - automounting and airplay will certainly help me achieve most of what I'm after. Regarding a player I would probably use OMXPlayer; it is what I used on my portable player and it seemed to do the job :-)

You are right - we seem to have a lot in common with our projects ha ha!

I'm currently working on an aquarium monitor using a Pi and a few arduinos which is letting me improve my Python coding no end - I'm hoping it'll definitely give me something I can use on this project!

For the hardware, I'd get a Pi and some USB speakers, as well as a Bluetooth adapter, and experiment away!
Do you need some kind of an interface? I think I'll have just a thing for you if you do =)