assuming the sub is amplified, and working the shows the ICC is transmitting audio. If all speakers stop working at the same time, for it to be a wire it would have to be power/earth related at the factory amp or an aftermarket (if it has one) each speaker is driven individually so there will be a common point.
If it has aftermarket amps it will have RCA leads and HLLC hidden somewhere, that's where I would be looking.