I have been looking at these boosters, and was confused by the 10' distance requirement between antennas. I too wondered, how can this work in a car. It is nearly impossible to get 10' of separation in a truck camper. I had not seen this thread until today, and now I am thinking this will be my next mod project.
I have seen some RV'ers use a Wilson directional antenna, on a pole. More trouble to set up, but they report more gain than the vertical stubby. That would be the cat's meow, but I am not sure the directional antenna is type accepted with these units.