[sinbad]

Cymro is right about the air gap between the white plastic panel the speaker mounts to, and the grey plastic enclosure. This needs to be sealed in order to isolate the front wave from the rear wave. I have not done so, but I think that you could use some strips of "Dynamat," that adhesive-backed rubber stuff, available from Crutchfield.

 

[sinbad]

Another solution for the rear speakers: Does anyone make a small shallow speaker cabinet for a 6.6 inch speaker that is open in the back? I am thinking about the same size as the OEM grill. You could mount this directly over the hole and it would seal around the edges, and therefore give you a nice big sealed cavity behind the speaker.