You need the Radio Removal tool, its made by Becker but you can buy them on ebay.. i like these.
http://cgi.ebay.com/ebaymotors/Audi-Mercedes-Vw-Porsche-Radio-Stereo-Removal-Tool-Keys_W0QQcmdZViewItemQQcategoryZ35625QQihZ011QQite mZ320209361197QQrdZ1QQsspagenameZWDVW
Of you can drive over to a radio installation shop and they will pull your radio out for you for free, just as long as they arent asked to do more than just use there Removal keys to pull the radio out.