As above, I'd highly recommend the Mr12volt box, superior sound quality via the digital fibre optic network (MOST) as opposed to the anologue AUX input of all of the other boxes available. Integration is much better as well and looks more OEM.
I just happen to have a video on how to install it (direct link in video description)....
I've installed a few of these Mr12volt boxes and I've installed quite a few of the other type as well (I used to run one of the other boxes for quite a while until Mr12volt brought theirs out)
Here's my video on installing the other type of box (they are mostly the same in conectivity)...
Hope that helps