Was the factory HU ever good?
I would just plug it back in as well, though since my HU isn't installed great,, I'd plug it up to the battery with some connectors,, but I'd be real careful ,,,, I did a school science fair project one year, used a 12v battery to power a bunch of LEDs, it was funny when I wired a few up wrong,, amazing how much smoke poured outta the little things and the bit of flamage as well.
