It has been my experience that the positive is voltage, the ground is chassis and the third wire goes to the meter.
Didn't they give you wiring instuctions?
If there is a biasing resistor or box, it would be called out in the instructions.
Biasing would allow an accurate reading over a wider voltage range like 8 - 16 V.
This means the meter voltage would be much lower...hence not pegging the needle.
Just my worthless opinion.
