If its a new Polo then pop down to halfords and ask for audi/vw release tools which they sell on the car audio section for around £4, this will then take your unit out, if its older then again halfords sell the release keys(if they dont know say the U bend ones that Vauxhall/fords use), if its a polo with a Sony unit as standard then you can do it by poping a screwdriver x2 quite thin around the top left and right of the unit between the unit and security cage and this should then release it for you:)
Rich
January 2005