Shorted Stereo System?

[Niall]

I reckon you have pinched a cable putting the stereo in
The cage sits on a plastic dash so placing the stereo into a metal cage cant do any damage because it cant short to earth...and even if it did, you then have to question why is the casing live.
You may also have a shorted connection on a speaker. I know my Kenwood HU needs disconnecting from the battery for a minute or so if it enters its protection mode if it sees a shorted speaker cable.