If you are replacing the OE radio, then when you removed it from the dash, you should have had to unscrew 4 gold colored screws and removed the radio and the small pocket as an assembly. If you look at the side of the OE radio you will see that it has two screws in each side. You should be able to uncrew those and the OE head unit should slide out. Then you can take your new head unit and slide it in. Most, if not all, aftermarket decks have threaded holes in the sides and you should be able to line up a couple of them and bolt your deck into the factory bracket.

You may have to play with it to get the depth right. the pocket is held to that same bracket by a couple of screws and you shouldn't have to remove it. Then you just slide the whole assembly back into the dash and bolt it back in.

It is a very secure mount and it is really hard for a would be theif to steal it. Believe me, I came out one morning to a busted window, no speakers and a trashed dash, but the radio was still there. Had to have pissed them off.

Anyway, the sleeve that everyone is talking about usually comes with the radio and it slides up around the unit and in most vehicles you slide it into the dash opening and bend some little metal tabs up to secure it behind the tirm panel. If you use the factory assembly, then this sleeve is not needed and can be discarded.

Hope that helps,

Matt