You're going to want to run fresh lines from the battery.
From the MP3Car.com store I'm looking at buying new positive and negative lines.
What you'll need to do is lookup the specs on your parts and everything that you need to pull power. You'll want to figure out watts & amps. You already have your powersupply, the M4, so you'll want to make sure the watts you're pulling off that M4 matches up.
The AMPs you'll want so you can figure out the gauge and the fuse size you'll need.
Read through this guide, it will talk about wiring the computer in your car. It also has charts so you can figure out the wire gauge and the fuse size you'll need.
I'm now leaning towards 4 gauge from the battery to a distribution block. From the distro to the M4 I'm going to run 8 gauge. Here's a link to the advanced FAQ that you might want to read through. All my bookmarks for this are on my computer at home, but these are the ones I had handy from my worklogs.
I've just recently been working through all these issues on my journey with this project so I hope these links help.
Edit: You'll also want to see if your truck has an accessory slot, or a way for you to tap into the ignition. My Chrysler has an available accessory slot in the fuse box inside the cabin. I don't have the link for it with me that explains it but there are ways to find an ignition line so that you can have the computer turn on with the car.
The reason for running fresh lines is so that you don't overheat the smaller lines, burn out the insulation, and spark a fire with a bare line.
Final Edit: I forgot to mention you'll want to give yourself extra space. The chart says I could run 10 gauge with a 50amp fuse, and I rounded up already on that. It was suggested to me that I run 4 gauge from the battery to the distribution block and 8 gauge to the M4, with a 125amp fuse a foot within the battery. This will allow for greater power draw without the worry of whether or not I'd need bigger lines.