No pictures I'm afraid.... but as much detail as I can remember.
1. Source a secondhand or new mk3 elec. aerial
2. Retract the current aerial as much as possible
3. undo the metal clip at the base of the aerial on the outside
4. Remove the following screws - the the screw above the seatbelt shoulder buckle, the one below it, 4 on the unserside of the parcel shelf, 1 on the rooflining near the rear hatch and the one on the rear bootscuff plate nearest the corner. 0 pop the units out - there is 1 plastic retaining trim rivet near the seatbelt - pop the panel and pull the rivett out - and fix it back in to the trim panel ready top refix back on.
5. remove the rubber strip to the rearmost door (on the 5 door)to free the edge of the plastic panels
6. remove the rear light - take off the plastic cover, undo the 4 screws taking care not to lose one in the car - push the light cluster outwards to
the rear - and remove the connector for the electrics
7. Remove the pop-rivet plastic retaining clips to the side panel - this is where there should be a plastic strap holder on the side and the light for the boot - also any screws that are here
8 - the rear side shelf should now be free to lift upwards allowing the bit with the light in to be eased out towards the rear of the car
9 - disconnect the boot light
10 - you should now see through the metal lattice the aerial
11 - Undo the earthing strip to the aerial, and also the height adjusting screw completely
12 - the old aerial should fall downwards - allowing you to see - a) the electrical connection if it is already an electric aerial, and the aerial connection - disconnect both
13 - by putting your hand through the rear light aperture you should be able to turn the aerial to get it out through the hole.
14 - refitting is the reverse of removal - good ole Haynes terminology!! Dont forget to refix the earth, and if the aerial wasnt electric previously - there should be a white terminal block there ready to take the connections on the new one. - and make sure you have not disturbed the black water gulley pipe that runs through the panel