Not home theater but car radio. The radio in my car has line level outputs via 4 RCA cables to an external amplifier. The right hand side of my audio (front and rear) is stronger than the left. I am suspecting the radio because when I plug an rca cable directly into the amp both left and right channels are the same. 

I am trying to measure the voltage of the rca outputs from the radio but cannot get a reading. How do you do it?