I just installed a pair of Infinity 6x9's into the rear shelf of my R32, used the standard speaker covers, so looks totally standard... this is the method that I used...
removed the old speakers and amp, I bought some MDF spacers, 10mm deep...
removed the rear seat... 2 bolts and slide it up and out...
removed the carpeted section of the shelf... 2 plastic clips and a row of metal clips by the window...
now climb into the boot...

there should be a depression in each of the holes that the original speakers came out of, this will sit inside the spacer... (the speaker cone WONT touch this, even on full travel) drill throught the spacer and shelf, bolt the spacer to the underside of the shelf... screw the speaker to the spacer...
on the right hand side the Hicas control unit will be in the way, cut the spacer to fit around this, this speaker will sit about 10mm further to the right... but you shoul;dnt really hear the difference.
The power cable for the amp runs from the battery, under the sill covers, under rear quarterpanel covers the bottom of the seat and into the boot throught the same hole that the seatbelts go through... make sure the cable doesnt interfere with the seatbelts here... (5m power will get from the batt, to the rear wheel) i put a distribution block here, and ran the powercable along behind the rear seat to the amp, the phonos run the same way along the other side of the car...
hope that helps!!!
Ive always used 6x9's and a sub, along with components up front, usually pwered with 3 amps... may sound like overkill, but it sounds very good.. full control over the whole tonal range...