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Heavyparts, I feel your pain.... I don't have my manual with me, but there are three things I remember that affect this somehow:
The button that shows a speaker/headphones on the head unit - I think this might be the one. There's a similar button on the rear seat controls if I'm not mistaken (but I don't sit back there much so I'm not sure)
And then there are a couple of button combinations - if you press presets '2' and '4' (I think - check the manual) it locks out the rear controls. If you press presets '3' and '5' (again check the manual) it mutes something, possibly the DVD audio.
All of this is from my very poor memory, and it isn't easy to find in the manual as I recall so you have to do some reading through the stereo section. Good luck!
BTW, this stereo/nav unit is THE big thumbs-down on an otherwise great vehicle IMHO ('06 Expy Ltd). I wish they'd sell me a SYNC unit as a replacement.