I wouldn't bother amp'ing them in the rear...
I don't seem to mind rear door speakers as much as I did rear deck. Rear deck speakers, esp with tweeters are really distracting to me. When the speakers are in the rear doors, I don't seem to notice them.
I'd just go with full range drivers in there, no co-ax or anthing. Raw drivers can be found fairly cheaply at most audio shops.