No major reason other than I like the way the 6x8 fit onto the existing mounting bracket with minor modification. 6x9 would have required aftermarket brackets, which I didn't have time for.
I think that's just Sat antennas and/or USB. You have to add a Line out converter to it out for rca signal. Look at the other comments for the link to the video I posted on which wires I identified for my application. Keep in mind it could be different on yours if it's not the same unit.
