I just took my door panel off today. There was enough room behind that little grilles near the door handle, but it didn't look like anything to easily mount it to. Also, the 5.25" (or 6.5") down low will need a mounting bracket/adapter, probably with a spacer, since the factory speakers are extremely shallow and anything deeper than those will interfere with the window when rolled down. I haven't found any brackets that will work with it, yet.
I will be getting this setup to work eventually (with 5.25" mids and tweeters), so I'll let you know how I do it once I'm done.
Anyone know where to get an adapter/spacer for these things?