I've just replaced the rear speakers in my 2006 Polo GTI. Not sure about the other models, but the GTI has rear speakers already - split tweeter & woofer. They sound a bit crap though! Just replacing them with $150 Sony splits has made a huge difference in sound quality.
The rear panels are held on with plastic clips. I took out the seat squabs (bases) and started by pulling the panels inwards from the door openings. There's one fixing into the floor under the seat, just pull it up. The rest are into the side panel and just a sharp tug should release them. Remember to take all the clips out of the side of the car and replace them in the panel before putting the panel back on the car.
The tweeters (if fitted) are fixed to the trim panel, but the woofers are fitted to the car. Unlike the front speakers the rear woofers are screwed in place, not rivetted. Makes removal much easier.
You may need to juggle the seat back up and down to get the trim panel out.