Go to firestick and read their tech stuff as a start.

The left fender is popular because, from what I've read, its a reasonable compromise between performance and aesthetics for many vehicles. There are some "rules" about how far you want to keep it away from the windshield to get good results. Too close and the upper part of the body blocks the signal in certain directions. Too close to the front and the ground plane business means that you'll have more signal strength toward the back than out front. (Maybe if you're the leader of the pack, that's what you want. <img border="0" title="" alt="[Big Grin]" src="images/icons/grin.gif" /> )

The middle of the roof is the best place for many vehicles, but most people don't like drilling holes in the roof to mount an antenna. There is also the clearance issue - got to watch out for garages, trees, etc a lot more.

I'm not sure I understand what was being suggested for a glass mount, but I don't think I've heard of anyone actually recommending a "through-the-glass" antenna or the "share the am/fm antenna" gizmos as good solutions.

I'd suggest a web search for cb antennas. There are lots of issues and even all the info out there still leaves many questions unanswered.