The 5.1 issue: do NOT use a center channel speaker without a 5.1 processor...it will sound like SHAT. Been there, installed that, customer even hated it. If you feel the need for 5.1 in your car ( I don't know why...) use a single-din speaker like NIXmx suggested. I know Alpine makes one.
Tweeter placement: I just drilled a 3/4" hole into the door of my supra and mounted the tweeter there. As far as perfect imaging, the best thing to do would be to put the tweeter in the kick panel. Don't put your tweeters behind you. Hell, don't put ANY speakers behind you for imaging that's better than "piss poor". And PLEASE...PLEASE don't power a component set off of just the headunit's internal amp. Component sets--or at least any that are worth a shit--require at least 100wrms, usually more around 150. My Diamond Audio components only "require" 100wrms, I'm driving them with 200wrms of CLEAN power (most important word: clean) and they sound phenomenal.
For great imaging, spend your money on a good set of components, placed up front, with a good amp to drive them that puts out CLEAN power. Adding a sub is also a must for great imaging, 6.5" speakers just cannot put out the lower notes that even an 8" subwoofer can.