Rostill - piece of piss, but only if you know what you're doing and have the stuff. Problem with mk3 Golfs is that the glass is what we call "encap" which means it has a trim bonded to the glass. In the case of the mk3 Golf, it's a flap of rubber around the perimeter of the glass. On the side that bonds to the car, it has a raised section of rubber (part of the flap you see from the outside) that is the height block for the glass (so it sits at the right height from the aperture)

To remove these without cutting this rubber encap from glass isn't the easiest thing in the world if you haven't done it before. But that don't mean it can't be done

From the inside, strip out the trim around the window - rear quarter panel "door card" and the surrounding trim. Then, you need a SHARP craft knife, like this one : http://www.olfa.co.jp/en/body/detail/15.html - but don't use segmented blades, as single piece blades are much stronger.

Get some glass cleaner (the blue stuff you fill your screen wash with) and put some in a spray bottle. Spray the shit out of the bond you can see from the inside of the car (I mean SOAK it - you'll probably need to keep doing so as it'll evaporate)

Then you need to VERY carefully cut through the bond with the knife. You need to stay as close to the metal of the car as possible so you cut the bond, not the encap. Be carefull not to have the blade too long as a) it'll snap and bend easily and b) you don't want to go through too far and scratch the paintwork outside the car or cut off the encap flap. Look at the glass from the outside and visualise a 1cm wide section all around the edge of the glass (not including the encap flap) this is where the glue is!

Now if you are carefull and slow, you'll get it out without destroying the encap. Quick tip - get some masking tape (make sure the window and car is clean!) and stick a few strips (good length for strength) across the window to the car body - say two vertical strips and one across the middle, horizontally. This will stop the window falling out and smashing on the floor when you cut through all the glue!!!! (Wouldn't be the first time I have seen that happen )

Hope this helps