I just had this problem the other day on the 4800..this is how it was on my 4800
1. turn off headunit, ofcourse have the car on.
2. hold down the source button untill the clock appears
3. press the function button to change it from showing the clock setting to aux setting.. from there you just turn on the aux, simple.
by the way, how do you like the 6800, i'm about to get the 680 (premeir version with 4v preouts)