Air Conditioning Vents

I have read pages and pages of all the great mods each of you have done to your Cat. One that I want to do is to replace the air conditioning vents with ones that swivel and close, and maybe install a small fan inside that I have seen some do here on the forum. My question is, how do you remove the old vents? Mine do not have screws so that I can remove them, and for the life of me, I can't figure out how they are removed. Any suggestions?

Maybe you have a trim ring installed ?

Here is the site where most of us bought new vents: http://dwincorp.com/index.php?cPath=55