Received my Zumo 390 in the mail today, using the supplied ram mount I have it secured to the clutch bracket. Because I am some what lazy I am going to wire it into the left side cubby. my question is I hear talk of people being able to use the audio 3.5mm plug to hook their gps to the speakers. My 390 does not have a 3.5 jack, just a usb port in the back, am I sht out of luck ? Thanks in advance