I've got a Miglia Eyetv, it works well stationary, but when you start to move the picture/audio breaks up. Tried a better aerial/cable positioning etc, and still can't get a decent picture on the move. I don't think it is possible, especially with digital (which it is). A pity really as you can also get radio through it.
Working for a couple of different car audio store we ran into the same problem with purpose built 12v TV tuners. Stationary you could get decent signal but while in motion there were problems. We did see gains with different antennas, for the life of me I dont remember the best ones, but I think it was with the 30" lg (or so) antenna that looked like the old glass mount cell phone antenna.