Non DSP (HK) factory amp- which pin is the ground wire??

[01whitex5]

I'm adding a sub and amp to my x5 and the High Level Output wires for my subwoofer amp have been tapped into the corresponding speaker level wires on the factory amp wiring harness. Among the R+ R- L+ L- wires I've tapped, there is a Ground wire as well. Does anyone know which is the grounding wire in the factory amp wire harness? (There's one plug on the factory amp, consisting of a black plug and grey plug)


I tried "grounding" this wire from the sub amplifier to a metal bolt but speakers did not work (though sub did), so I figure I need to make the correct ground wire tap in