USB connection new car stereo 5 wires to 4 (schematics included)
The signal "shield" should be a foil or metal mesh wrapper around the four signal wires.  It is there to prevent interference from the environment, which could make the link less reliable. If there is no connection for it on the new head unit, you're probably best off connecting it to the metal case of the head unit instead. You could also try connecting it to USB GND at the stereo end.

As for wiring the other wires: red, green, white and black are the standard colours for USB, in which case the assignments given in your last paragraph make sense.