Very interesting. Usually there is a ground bus somewhere under the dash where all your electronics are grounded. Follow one of the negative wires from one of your gauges and you should be able to find the ground bar. It should have one terminal straight from the battery connected as well as a bunch of smaller wires connected to it.
I would try grounding both your amp and head unit to that.
Also try running your HU power to the same place as the amp.
Your speaker wires should be going to the amp and not ground.
(Message edited by l1spoogy on May 31, 2005)