The rear speakers are actually oversized 6 1/2". 6x9 will fit. I thought the same thing when I upgraded mine. All you have to do is screw in one side and put a zip tie on the other. your right on the door size. New speakers will sound a bit cleaner plus they'll last longer. I have 6" 3 way pioneers in the doors and 4 way 6x9 in the deck. sounds great. The factory tweeters work fine. Granted the stock system sounds great and will shake the rear view all day but you can always do better. I've never cahanged the head unit because you have to re wire the whole stereo. The reciver is in the trunk on the passenger side fender. A aftermarket double DIN wont work. Trust me. it took me a week to figure this out. You also have to cut up the dash to make it fit. I've seen standard DIN units installed but you have to fill the hole left by the stock unit. And agin you still have to rewire the car. I just added a 12 disk changer and I've been happy with that. Unless you want some kick ass stereo it's not worth it.