this post on my build. Basically, the 8 wires you get are In1, Ground, In2, Ground, In3, Ground and In4, Ground. They correspond to 8 output channels - Out1, Ground, Out2, Ground etc. Basically, you can use:
Input 1 - Front Left
Input 2 - Front Right
Input 3 - Rear Left
Input 4 - Rear Right
...in a 4 speaker configuration, and make sure outputs go to appropriate speakers in your car. So in other words, you use 2 headphone jacks, as you can see in my picture. If your motherboard only has 1 output, you can merge Input 1 and Input 3, and Input 2 and Input 4, so both Front Left and Rear Left speakers will play the same sound, same as Front Right and Rear Right. If you do that, you only need 1 headphone jack, and you would just plug it in directly into the motherboard. Alternatively, you don't merge inputs, and use a headphone splitter from radioshack that would essentially do the same thing.
amplifier thump. This amplifier can be powered from an unregulated 12V source, like your ACC line.
2 last questions,
Only because Im not great at soldering and splitting wires, can I just cut the 12v and ground from a Molex thats connected to the m2-atx to power the tda7850? wouldnt that work and also not require a relay because the m2atx would power the computer first, if I still need a relay, where did you get the relay for the amp?
Yes, you could certainly do that. In fact, using a regulated 12V line is better than unregulated. However, you need to look at the total power consumption. I've put a 5A fuse on the amp line, and it hasn't blown, so it's safe to assume the amp will use less than 60W (12V * 5A). M2 can give up to 140W, so that leaves you with 80W. If the rest of your system can be safely powered by that, I'm sure you'll be ok. I wouldn't push the power supply to its limit though.
You might or might not need to use a relay - it really depends on your hardware. I used a relay because when the AMP was powering up, it would make a loud static hiss noise that would go away in a second. Because of that, I made sure to start it muted, and then unmute it by using a USB-controlled relay. You can find that relay on ebay - just type in "usb relay".