I've got my VHF and UHF Motorola speakers mounted on the rear package tray, so they fire upwards, next to the rear stereo speakers. The sound is reflected off the rear window and into the cabin very nicely. Some police agencies do this (Michigan SP) and it seems to work well. It solves two problems: Where to mount them out of the way, and how can I still hear them? I also have my UHF vehiclular repeater antenna mounted on the rear deck between the speakers. The nice part is I can throw a couple baseball hats over the whole mess if I really want/need to be discrete for some reason.
I had considered mounting them on or near the B pillar, which I had done in a previous new car, but my two current vehicles are laid out in such a way that my head would probably be hitting the drivers side speaker all the time.