You should have at least one light lit or flashing on the PB4. If not I would start there. Without the head unit connected to the PB4 the status light should flash. This indicates there is no buss communication with the head unit. If that works then connect the head unit and see if thr status light is lit all the time. If the status light does not flash when PB4 is powered up without the head then the issue is probably the PB4.