Antennas like to be above things, not beside it or under it. Getting that antenna outside the metal body of a vehicle is a very nice thing to do. It isn't a 'fool-proof' way of hearing everything you want to hear, there's just no 'magic' way of doing that (no matter what the advertisements say).
How a signal get's from here to there is called propagation and is determined by a large number of thingys. Sometimes it just won't 'stretch', you know? Buildings, distance, terrain, 'Momma Nature', they all have a finger in that pie.
Noise is a fact of radio life. There are so many ways for noise to enter the picture, it can be very mysterious! And another little 'goody' is that vertically polarized antennas tend to 'hear' more noise than any other type of polarization. No way to get rid of all of it, but you can get rid of some. Find the source of that noise and eliminate it, or 'hide' it from your radio. That's so much fun! Right?
Oh well, do what you can and then live with it. Good luck.
- 'Doc