I had trouble with that also. You need a 4mm or a 5/32" allen wrench and it is normal right hand threads. You can see were to engage the wrench through a small hole behind the weather strip. It has been a long time ago, but in your picture try removing that rubber grommet and look through that hole. As I recall it is not perfectly vertical, you may have to move your allen wrench down a little. The longer the wrench the easier it will be for you.