You mean something like this?
These 6.5" subs are supposed to sound real good for their size. I have not heard them yet, but have them installed in the doors of my '05 SE KC.
I will be changing this up a bit, and recessing the subs a bit and fabricating grills for them.
As for 8" , with the bracing in the doors it will be hard to fit more than these 6.5" subs. There is not a lot of air space in there.
Check out
Elemental Designs EU-700 Subs
