Best way to plug in audio jack ?

[Jvt]

Hi.

I have a Yaris TS from 2001. I bought it 3 months ago, it is my first car, and I really like it! :)

What I like a bit less is the old cassette/audio-cd system. I'm planning to install a car computer, using a Raspberry Pi (http://en.wikipedia.org/wiki/Raspberry_Pi) that I just bought, and a touch screen. Maybe I'll add a reverse camera, a GPS module, a USB Wifi adapter, ... So it will cost me some money, but I want to do it for the DIY fun.

I'm kind of good with computers, but not so much with cars... The first big problem that I can see is for the audio connection. I have two options to get sound from the computer :

A 3.5mm audio jack port.
An HDMI port, that will be used to connect to the screen, but from which I could also get the audio, using a device like this one (http://www.befr.ebay.be/itm/HDMI-to-HDMI-SPDIF-L-R-Audio-Converter-For-Blue-Ray-PS3-HDPC-DVD-Home-Theater-/191074759433?pt=US_Splitters_Combiners&hash=item2c7cf17309).

Now I have no idea how to plug it to my car speakers, and that's why I thought you could help me. What are the options, and what are their advantages/disadvantages ?

Here is a picture of the center console (http://i.imgur.com/F8ZP9mN.jpg?1), if it can help.


Thanks.

[Jvt]

No answer ?

By the way, what I'd like to do (if it's possible) is to remove all this part in the bottom, and put my 7" screen there instead :
http://i.imgur.com/pyMYj2P.jpg

About the audio problem, I found two possible ways to do what I want :

Using a cassette adapter. Apparently these things work better than what I'd expect. But it means I'd have to keep the cassette player. Maybe I can try to hide it somewhere. At least I can remove the CD player, right ?
Using this thing (http://www.befr.ebay.be/itm/321363674772?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649). But how does it work ? Can I get rid of all the bottom part of the console if I use this ?

I'll buy the cassette adapter anyway for now, it's cheap. But I hope I can find a better solution.


So, another problem is that I'm going to need power for all these things (computer, screen, ...), and again, I know almost nothing about it. I wouldn't dare to ask you exactly how to wire each thing, but could you please provide me some basic understandable documentation about how to wire things to the car battery ?

Thanks.