I'd like to extend this thread a little, hope you don't mind too much seanman.
When you use the high level inputs and tap your existing speakers, does it rob a lot of power from them? Also for wiring is it always done in paralell with the existing speakers...if so, does that mess with the load that is now drawn from your head unit (by the speaker and hi level input combined), which in turn has adverse effects?
Sorry about the jack, but i've been wondering about this.