No this will not work like you are thinking.
Your Aux output has only 2 speakers worth of connections. There is no way to magically make your 2 speaker connections (3 wires) grow into having completely separate connections for 4-6 speakers. All you can do with the Aux is use a splitter to clone the same signal to other speakers.
At the end of the day hackers/modifiers buy the off the shelf equipment because it is not only easier but also more affordable. By the time you throw in the cost of the amps, the converters, the wires, and not to mention a voltage regulator so your fluctuating 12v dc system does not fry your pi you end up spending more money then even a double din touchscreen head unit.
If you are dead-set on using the pi for your car audio you have 2 options:
1) You can just use Aux a splitter and some amps if you are perfectly ok with the front and rear having the exact same music.
2) You can use hdmi to get you more audio channels and use a device like what is linked bellow to then convert it to rca to be used as in input for front and rear channel amps.
http/www.amazon.com/ViewHD-Extractor-Decoder-Headphone-VHD-H2HADi/dp/B00L0XIHEC/ref=sr_1_5?s=electronics&ie=UTF8&qid=1445290454&sr=1-5&keywords=hdmi+5.1+audio
Now with that said, if the audio source is not multi-channel then this setup will only play over the front L and R channel unless you find a software audio codec to push it to the other audio channels.