that metal cage you're talkin about mounts to a dash install kit, which you may have to buy since it sounds like you don't have one anymore. anyway, once you have that, put the metal sleeve in it, and bend the little tabs (on the metal sleeve) down to lock the metal sleeve in. it shouldn't move once you bend down the appropriate tabs. after that, the headunit (assumming its an aftermarket headunit) should slide into that metal sleeve, and catch itself at the end and lock itself in place. when you buy a new headunit, you'll get a metal sleeve with it.
sorry i dont have any pictures.. it's an easy install though, so if you have any questions, there will be lots of guys on here that could probably help ya out
2004 Ford F150 FX4
Mods are here and there...