Hi, Mark.
Your problem is very obvious. Battery can't handle your stereo system
because
of the power wire which is another major current draining from
battery. Battery
is not designed to handle too much current going through power wire.
The power
wire is big fat cable requirs big current going through. Your battery
can't handle another that..

The solution is: disconnect power wire. Connect amp with a normal
+12volt source.

Mark, please don't listen to this guy. He's been trolling this newsgroup
for a few weeks now. During that time, he's exhibited a high level of
ignorance, bias, and racism. If you've got a killfile, he may be a
worthwhile addition.