BMW XMK1200LT Motorcycle. I have an 04 bike with single CD player above the fuel tank and have a problem with the radio. Only 1 speaker out of the 4 works, this is the right rear. I've tried each speaker in turn in that position and they all work. I have 5.6 volts (approx) on all speaker wires to earth. Can anybody suggest what to do next please. Rob