Haven't had to deal with the grommets yet, the one I got from a parts car was rotten out, I'll be tackling that tomorrow. As for the assembly itself, held in by a screw at the bottom. Large phillips, probably have to push on the driver with one hand and turn to loosen it. Pull it down and out, unscrew signal to radio, unplug connector for power and done.

Opening the assembly itself isn't too bad. I found it easiest to pry up one of the long sides, work my way down around the rounded part, then finally do the two larger tabs at the flat top.