Now for the rear speakers...
The stock rear speakers are recessed into the spacers and look like this:
Remove the rear speakers from the spacers in the same manner as the front. You'll be left with this:
The rear spacer has two parts which I'll call upper and lower. The upper part faces in toward the cabin; the lower faces outside. If you look at the upper part, you'll see the spacer cone. If you look at the lower part, you'll see the magnet. They are glued together but can be separated by scoring where they connect with a sharp knife like so (in this picture, the upper portion is on top, the lower is on the bottom):
A flatbladed screwdriver applied along the cut helps to separate the upper and lower part of the spacer. Once that's done, you'll have two pieces like this:
Cut a 1/4" off of the bottom of the upper spacer (this is about the thickness of the rim of the new speaker). Here are the beginnings of the cut:
Follow the same steps for mounting the front speakers to the spacer to mount the rear speakers to the lower spacer. Once you have that mounted, invert the outer spacer such that the part the was facing toward the cabin is now facing outward (touching the screws holding the new rear speaker to the lower spacer). Mark where the screws are and drill. Unscrew the speaker from the lower spacer. Place the upper spacer over the new speaker and lower spacer. Screw all three together and you'll get this:
Since the outer part has been inverted, there's not much to attach the outer foam gasket to. I used some super glue to help secure the gasket to the upper spacer like so:
Now just solder the connector to the new speaker and you're all set.
Here's a before and after comparison:
Here are all four of the speakers after the mod:
I tested the new speakers after installation and didn't notice any vibration or distortion. I played music through each speaker individually at max volume with bass, midrange, and treble separately maxed out with no problems. Response is good across all frequencies as far as I can tell. I listen to classic rock and indie for the most part, so I'm not pushing anything too hard. This is all with the stock head unit, too. You get what you pay for, to be sure, but I'd say I got my $100 worth.
Any suggestions on how to improve this setup are welcome. The fit up front is a little tight, so that's a good place to start; it didn't appear to affect speaker function, though. The rear spacer mod is a little contrived, but it's the best I could come up with. I tried to avoid excessive fabrication, gluing, and spending money on anything beyond the speakers.