I have the same problem. Iphone 3G here on 3.0
Tried the 50.00 dock cable and a 1/8th cable on the top of the iphone, neither give me video. Have not tried it on my home tv yet.
Makes no sense, if video goes out of the ipod through the cable and works on your tv it should work in the truck. Nothing is changing from the video delivery point of view.