I fitted a DAB/FM/GPS shark fin base off a Polo R on my ED30 a while ago --->
http://www.mk5golfgti.co.uk/forum/index.php/topic,69555.30.html It's one of the last pics if you want to see it.
Straight forward job to be fair.
Open boot
Remove plastic trim in the center of the roof (pull it gently)
Look left and you will see a bolt holding the C pillar trim - remove it
Look right and you will see a bolt holding the C pillar trim - remove it
CAREFULLY feel under the roof lining and find the arieal base and run your fingers down the cable to find the connector
Disconnect the old aerial base fakra connection
CAREFULLY undo the aerial base nut - can't remember exactly but I think it was a 14mm
Remove old base
Clean where old base sat
Insert new base
Don't over tighten the new base as it could twist the roof skin - make sure it's straight
Connect everything and replace trims