some on has done it mate, he also wrapped his car to a really light blue but for the life of me i cant remember his name :facepalm
Open the tailgate, remove the boot appeture seal, remove both rear "c" pillar trims and upper seat belt bolts, remove the rear interior light from the headlining along with the grab handles, remove the trims as needed, gently ease the lining down put your arm through with a 22mm-24mm spanner and undo your old aireal and pull it through gently, disconnect the aireal, refix your new one and put it back together!
The shark fin aerial if only designed for telephone and Sat Nav comms IIRC. The radio aerial on 8P cars is integrated into the rear windscreen so you won't get much of a radio reception by fitting this to an 8L. I think the owner acknowledged this in his thread but did it more for cosmetic reasons. I agree it suits the car much more but sadly won't work too well. I think there are alternatives to the standard long aerial but avoid the cheap ones on ebay as they're rubbish.