Theres two ways of removing it, if its pre 95 its a bugger. U need 2 very thin screwdriver/blade type tool which you place down the middle of the radio either side being carefull not to scratch anything as you will see it all when you take the radio out and put the new one in.
If its after 95 it will either have 2 holes either side or 1 hole either side, halfords sell the ISO release keys for it, place the keys into the holes. It can on the pug be tight some time so use a blade/screwdrive underneath to help prize the unit out. If you get the keys and your radio only has one hole either side just open up the release keys.
Rich, January 2005