I try to just pick whatever spot I can fit the radio. Sometimes I’ve tried to buy specific radios, like the little “paperback” radios that I have or because in my green Ranger the best spot I’ve been able to come up with is inside my custom center console and thus will be rather inaccessible, I’m looking at getting a radio with the controls on the mic so I can hide the actual radio.
There’s really no right or wrong answer for where to mount the radio, whatever works for you. I’ve sat in the drivers seat and held the radio at different possible positions and tried to find whatever didn’t get in my way but left it easily accessible from the drivers seat.
A decent CB shop should test things for free or a minor charge. It’s potential business for them if something has to be fixed/replaced/modified. The shop I went to, they tested stuff for free, then I ended up buying, if I mind right, a new coax, mount, and my first 102” whip. Then knowing what I needed, I bought more of that stuff online later to start outfitting the rest of my fleet.