I replaced my front and rear door speakers with $60 Kenwood 5x7's from Crutchfield. They aren't the highest quality, but they sure beat the paper cone on the factory premium speakers. If you order from Crutchfield, they will send you the wiring harness free. The rear doors are as simple as disconnecting the old speaker and attaching the new one. The screw holes line up and everything (if you buy a 5x7 made to fit Fords). My wife could do this installation.
The fronts were just a little more complicated. If you have the tweeters in the A-pillar, they are wired into a crossover that is attached to the 5x7 in the door. I was able to remove the crossover on the first one, but the second one broke. So I went to my local car stereo place and boughts some bass blockers for $8. I wired these between the door speaker and tweeter, and all works well.
I have the 4 door speakers and 2 tweeters powered by the factory head unit. My sub in the trunk has a separate mono amp. I don't plan on changing the tweeters unless I found some for a great price. Hope this helps