Flip the plastic cover up, then undo the bolt - then to get the old one off, it is best to use a tool called a 'puller' else you may damage the rear window.
A local garage/friend may have the tool (brand new £12 in a shop, prob £5 on ebay) looks like two pronged claw with a bolt through the middle.
Takes less than a minute
cheers
Colin, February 2009
