And by the way, sometimes when I've done this, it just took a lot of working at it with a screwdriver (from the inside of the sled) until I finally got enough "bite" on the tab to do it.

I don't know what kind of car you have, but is the dash plastic (the part that the tabs are attached to) removable? Sometimes you just unscrew a few hidden screws and the whole assembly pops off without trouble. Then you can get to everything from the back. It makes installations and deinstallations a lot easier if you know how to remove the dash. Have you got a Crutchfield Master Sheet for your car? Can you order one?