Replacing speakers rear shelf

I think it depends on whether you're rear seats fold down or not.
Mine do so it was a simple case of disconnecting the rear brake light, popping up the rear deck and sliding it forward and out. Not sure what all the other removal stuff was about as I didn't have to remove anythig other than the deck cover itself.
Can't see how standard speakers can be screwed into the OE locations without an adapter as the profile where the speaker meets the rear deck is not flat (it goes up & down thus leaving air gaps all round the speaker).
I used Pioneer custom fit units, after trying to make adapters, although you can now buy adapter rings for the rears now.
 
you can get adaptors from a car hi fi shop. In my old A4 B5 i put caliber speakers in the adapters. The adapters are about 15 pounds i think.
 
They are dead easy to change,adapters screw in and speakers fit to them.I ended up taking mine out to help vent the bass into the cabin