Jim's right on the money with his analysis. Following his instructions and
it will help you find the cause.

It it ends up going away when you unplug the stereo, it could be a grounding
problem of sorts. If your radio and amp are grounded at different points,
you could have what's referred to as a ground loop. Try tying the amp at the
same power leg as the radio (both power and ground) to see if it goes away.

If unplugging the radio doesn't help, you could try to move the power to the
amp closer to the battery (ideally, on the battery posts but that may not be
convenient).
Before you do all that, you may want to make sure that the hiss doesn't come
from the amp -- just get a separate supply (another battery, if you have
one) and see if hooking the amp's power just to that battery and nothing
else helps at all. It could be inherent of the design, but I'd exchange your
amp to make sure.

If moving the wires, etc, does not work and the problem is not within your
amp, you could try a radio shack noise filter (Catalog #: 270-051). I don't
think it will be able to handle the full input power to your amplifier,
though, as they are rated at 10amp - check your specsheet.

Hope this is useful.
Remco

Problem can be fixed easily by adding a floating ground available at any
audio shop or Best Buy.