I'm pretty sure with the windows rolled down, you got about 3 1/2 inches back there. Most good 6.5" door speakers are gonna be shallow mount anyway. I've got a pair of Boston Pro SL-60 componenets in my door, and the sound quality is superb. Got the crossovers stuck to the inner door skin and the tweeters mounted on that lil sail panel piece of plastic that covers up the bolts for the mirrors.
Just remember....you get what you pay for.