My car has just about everything people usually do mods on installed already...
(OTIS, AutoStick, ATC, 60/40 rear seats, etc.)
...so most of this will be just cheap non-stock things.
Rear Speaker Grilles:
After a few hours of work, I was able to get the back of my car all taken apart and get the rear deck cover out. I'm not as strong as I'd like to be (I barely weigh 140lbs...), and the back of the rear seats was just a PITA. Once I'd get one of the hooks off, then try to get the opposite side off, but whilst doing that the hook I just got off would fall back into place... ugh.
Rear Deck Cover
My dad had some speaker grilles from a couple 6x9 Infinity's he unfortunately blew out, and he let me use them for this. My first thought was to just try and use the grille without the silver trim it came with...
Tracing the grille
Cut big enough for the grille
Opposite side... the grille wasn't gonna be able to cover this up good!
After widening the hole, got one in!
Aaand eventually both of em
In the car
It sounds fantastic. It's as if there was 4 layers of cardboard covering up the back speakers, the difference in mids and highs is night and day: it sounds like I installed some tweeters back there. As bass isn't directional, I haven't noticed much of a difference with that, but if there was any more bass back there it'd probably be overkill anyway! (If it's enough to fling chunks of ice off my windows in the winter, it's plenty.)
I didn't get any pics whilst installing this, so I only have the pics of it installed to show. I didn't do any while working on it as I didn't expect it to work right away, I highly doubted my soldering skills. It's spliced into the CD changer, if there's nothing plugged into the jack the CD changer passes through as usual, but if I plug something in there it'll play instead of the CD changer.
Note where the flashlight is pointing...