Sorry for my delayed reply......
I went the entire aftermarket route, replacing all of the speakers, including the rear sub, and installing the extra tweeters in the rear, all powered by two new amps installed under the two second row captan chairs (sounds way better than stock BTW).
But if you don't want to go that far, I think using aftermarket tweeters with a separate amp to power them, is the right way to go. The tweeters will not provide any mid range back there, but the sub located back there will help to offset the lack of any bass from the tweeters. As I posted above, I was amazed at how much better the system sounds with the tweeters in the rear. It didn't matter whether you were sitting in the 3rd row (where the biggest difference was heard) or even up front in the drivers seat.
I did the install myself (I bought all of my stuff from Crutchfield & Sonic Electronix). It was not that bad, it just takes time. If I were only adding the rear speakers, it would have been less work than re-doing the entire system as I did. For what it is worth though, I would recommend replacing the factory speakers with aftermarket. Even with your stock head unit, the system will sound so much better and replacing the speakers with the stuff from Crutchfield is pretty much plug and play.