For those wondering about the possibility of cable losses in a long cable potentially eating up all the gain from an external antenna mounted as high as possible, yes, it can happen.
To illustrate, back in the days when CB radios were all the rage, an acquaintance who lived on an acreage a few miles out of town was right on the limits of being able to talk with CB users in town from the base station in his house. He had a good directional antenna mounted on a short mast right beside his house. The bottom of the antenna was just above the roof.
He decided he wanted to improve the antenna performance, and, of course, everyone said, "Get it up as high as you can."
He lived a little bit down in a valley and the back of his property was at the top of the valley, so he got a longer mast and set it up at the top of the hill. The only problem with this setup was that he needed over 200' of cable to get from the antenna to his radio room in the house. He was rather disappointed to discover that he had no better performance from all his work.
I suggested that he just move the taller mast back beside his house, or put it on top of the house and drop 150' of the cable. That did the trick.
That's just to illustrate that there is a break even point. The frequencies we are talking about with GPS are far higher, therefore the antennas and all related issues, are much smaller. For instance, once a GPS antenna is two or three feet off the ground there's little to be gained from getting it higher, from a performance perspective. After that, the only advantage to height is to get it clear of obstacles that block its view of the sky.
The reason to get it outside on the roof of your vehicle is not to get it higher. It's because the vehicle itself blocks a large portion of the sky view when the antenna or receiver are inside, so getting the antenna or receiver up on the roof gives it a much better view in every direction. It also uses the car as a ground plane.
...ken...