The usb cable doesn't go to the radio. It plugs into the external Uconnect module which is located on the main dash structure. If you remove the glove box and look to the right you should see it there at the end of thee dash. This is for the REQ, RES,RHB & RBZ radios.
I bought a 2012 3500 ST and later saw a USB port in a cab at the wrecking yard while looking to get parts to install an upper glove box. I pulled out my upper pocket and the USB port was already there taped in behind the dash support cross bar. I plugged in the Ipod and heard a ding through the speakers but it won't play. I don't know if the USB is hooked to the radio. I have the Media Center 130 (RES) and Uconnect Voice Command w/Bluetooth. Hopefully someone can help me out. Pull your lower glove box then remove the upper and see if your have a USB already.