Guys the tweeters are easy to add. The midrange speaker has a full-range signal. The tweeters take almost no power, come with built-in crossovers, and you wire them straight to the midrange, and mount them wherever you want. Seriously it is easy.
My build thread shows you how. I used
JBL GTO19T tweeters, but you can use the equivalent from your favorite manufacture. The JBL kit is awesome as it comes with several different installation mounts allowing you to do the install however you'd like.
You will want the tweeters up front. You can install them in the rear as well, though they likely won't make a difference given their location. So in summary - replace the four mid-ranges, add the front tweeters (wiring shown in my build thread) and you should be all set. If you get adventurous, you can do the mid-bass footwell speakers.
Filippo