I've got a Dension Gateway 300 with my Symphony 2 and I'm playing the songs off USB.
Now, it's set to random via the HU which seems to be the only way.
However, it doesn't seem to be all that random.
When I turn the engine off and then load back up, it'll resume from where I last left a song and then the next song that plays will be a set song if you get me. I.e there seems to be about 5 different selections of "random" from the HU which plays a certain set of songs in order, turn the engine off and on enough and you'll eventually get back to the same loop.
This never used to happen when I used my iphone.