boss 3700

Installing a Boss PV3700 amp in my son's Durango. Ran all the wires like manual said, but the amp won't power up. Put my multimeter on the power and ground (key off - 0.00v), (key on - 0.00v); Ran a wire from battery ground to connect to my multimeter and rechecked (key off - 14.7v), (Key on -...