Some people have posted that they have mounted 6X9 speakers in their front doors so decided to try it. I used the shallowest 6x9's I could find. (Polk GXR69) I also like to use the foam "buckets" behing the speakers. By the time I was done, I had to top mount the speakers on the plastic door panels which meant I had to cut a 6X9 opening in the panels. I also had to trim quite a bit of metal from the stock speaker hole. I also had to fabricate the mount for the back side of the plastic door panel to hold the speaker in place. I was a major hassle but depending on how you feel about cutting your car up the sound in MUCH improved over the 6.5" Cervin Vega's I was using.
i've put 8" woofers into my doors, under the door panel. you just have to build a ring to space the speaker out far enough to clear the window. 6x9's are even easier. just measure and be patient, you'll figure it out.