Well, you could probably find an aftermarket module that would provide a USB port from the rear. However, you would lose the ability to control the ipod from the radio. The USB intergration is all internal to the radio itself. II'm not sure if the newer GMT900's include an auxiliary USB cable to the golve box but you can check, but I'm not sure, just giving you somewhere to check.
I have an aftermarket radio with front side USB. My solution was to buy a black IPOD usb cable, and run the cable on the edge of the dash and stuff my ipod in the glove box.
I have an aftermarket radio with front side USB. My solution was to buy a black IPOD usb cable, and run the cable on the edge of the dash and stuff my ipod in the glove box.