Set your meter to DC resistance, then turn the volume control all the way down.
There should be two pairs of pins that show almost 0 DC resistance between them. One pin from each set is ground, the other is signal + out.
Turn the volume control all the way up. There should now be two different pins that show almost 0 DC resistance between them. One pin from each set is signal + out, the other pin is signal + in.
Since signal out is present in both sets of measurements, process of elimination will give you ground and signal + in.
-PB