Get something like a 7in driver or an 8in. Hybrid has a 8in driver coming out, CDT has the 7in driver, and there are a few DIY drivers like the Tang Band 8in. You want one that is IB or atleast doesn't need an enclosure unless you're going to make a pod in the door for them. I'd have them play from like 60hz to 100hz.
I know very little about these types of speakers so I'll let you choose which ones you think would work. Maybe I should tell you a little about my system. I have four eclipse ti's in sealed boxes. I am getting diamond audio S600A's for the front components. I want to run the components and the midbass drivers off of a Kicker kx800.4. So which ever ones you think would suit me best. Do you take Paypal? Thanks alot.
Whats your HU? You need something that can bandpass the rear channels because you want the midbass speakers to drive between 80hz-250hz( or whatever your frequency range you want). I'm not sure if the Kicker can do that I know its has a high or low pass filter but not both on the rear channels.