At this point, it's got to either be the ground or a bad amplifier.
Can you check for power between the connection @ the amp, ie, ground to the ground wire, and check for power on the red wire? If you have a bad ground, that wouldn't show any power. You should get about 12.5-14 volts.
Derek Tircuit
Product Specialist
Bazooka Mobile Audio
1-800-THE-TUBE
derekt@bazooka.com