One option is to find a tablet with a good dock. Usually the docks will take care of both the USB and the aux, so you don't have to have wires sticking out of both ends. Also, the wires of the docks can be hidden behind the tablet.
Ultimately, for the cleanest install, you'll have to buy or fab up some sort of housing for that tray. If you build one, you can accommodate your wiring in it while you're building it, so everything comes out clean without any hassles.
The reviews on the screen is why i didn't get the Streak. It was cheap and had decent specs, but I didn't want to end up with a screen I couldn't use in the middle of the day.