I run firestiks on all my trail rigs and on my pickup, I have the antenna mounted to the back of my cap and run teh coax down, under the truck, through the fire wall. Here is what I have learned with Firestik antennas.
1. Do not cut the coax to shorten it, they are designed to use the entire cable that comes with the antenna.
2. 1 antenna will perform better than 2
3. try to get the tip of the antenna at least 2 ft above the top of the vehicle
4. get your self an SWR meter and properly tune the antenna.
As for the mounting of my CB, Mine is on a very simple bracket that is mounted just in front of hte center console on the trans hump. I am running a 29 LTD classic in my truck and by jeep and buggy both have 19DX that I got dirt cheap from walmart. I am thinking of putting a 19DX in the dash where the factory CD was, I have not yet measured or attempted this, but I think it would make for a more convient and cleaner mount.
Lastly, I have not seen a difference in CB transmission between the $100 CB and the $30 CB with a properly mounted and tuned antenna.