help with amp install

Just run rca's and cabling for a Sony 200W amp. The amp has been bridged and is running a Cerwin Vega 12"sub.  The source is a JVC single disc head unit. When the remote, earth & power cables are connected to the amp the amp will not turn on.  The 3 cables from the amp run into a female plastic quick connect joint which joins with a male one from the source.  I have tested the supply plug - when the head unit is on i get about 10-11V from the remote lead & 12-12.5 from the battery, all with repect to the earth.  The earth is run to the car chassis.When the supply plug is connected to the plug from the amp nothing turns on.  Testing the voltage at the terminals at the amp only gives about 3Volts - probably the reason why is doesn't go.  But when the cables are disconnected from the amp they all read about right again.  I have bypassed the voltage supply from the car with an external battery.  Connecting the power and earth from the amp to the battery, but still using the remote turn on supply from the head unit to turn the amp on - and this works! Can any body help me out! - why won't the amp turn on when the supply cables from the car is connected to the amp!. Note the car has just got a new battery