Simple problem. can't change brake light bulb on 99 cougar

HI, I have A problem. I''m trying to change the center brake light, i think it''s callled high mount light, on my 99 cougar.

the problem is i can''t seem to get the cover off. I got the carpet thing off the trunk and i depress the tab on the cover, but i can''t get it off. Can you help please.

thanks