When I received my 337, I searched too, and finally contacted GD for advice. They indicated there was a USB port on the Furrion stereo system that could be used for charging phones. I tried it, but for some reason, I couldn't get the power on the USB to stay on for more than a few minutes.
My solution was to order a 12V/USB docking station off of Amazon (see link below) and install it in the cabinet to the right of the stereo system. I drilled a small hole in the panel between the compartments, cut the wire to the docking station, and spliced it into the power for the stereo. If you'd like to do the same thing and have any questions, drop me a line.
http://www.amazon.com/gp/product/B000PB8CQI/ref=oh_aui_detailpage_o05_s00?ie=UTF8&psc=1
Jim