To be honest, I would highly recommend just buying something like the Ion Audio Tailgater, or go to a hardware store and get one of those construction site stereos like this one and use the AUX input.

I was never able to work out issues with mine with the audio cutting out every couple minutes. The power supply was a 8-16V DC/DC power supply that put out clean 12V (and 5V). I had it left over from a previous automotive computing project. That was overkill...I could have hooked the empeg straight to the lawnmower battery that I had inside, but I didn't want to risk ruining the empeg. The speakers were 12V also and got power from the same source. The battery charger was one of these things. Seriously though, just get something that's pre-made. None of those options were available at the time and we spent a lot more making our own than any of those options cost.