Okay here is my thoughts on this.

Why use the empeg as a source, this means you have a [censored] load of hacking to do and more fundamentally if empeg ain't in car you got no nasty noise. Why not try something along the idea of cheapo cd walkman with a custom made cd. You can already get a nice relay from Sony to swap RCA inputs.

For me i never leave the Empeg in its cage when i leave the car. It is either stuck in the boot or more normally comes with me but if it ain't practical to come with me. I have another method. get a redundant ISO plug like the spare supplyed with the empeg and leave that hanging out of the dash moron looks in car sees wires, pauses for mental processing, comes to conclusion, look wires hang out hole means some one else has already nicked stereo. Moron wanders down the street to next door neighbour with huge kenwood sticker in window.

If moron still breaks into your car you can pulp his mind with a really nasty low pitched wave, and a very high pitched painful one as well.