Your car has a factory amplifier unit in the left rear trunk quarter panel. You cannot connect the speakers out wires of a head unit to the inputs of the BMW amp, wrong signal/voltage levels. You must either get a head unit with line outs and an adapter cable, or a generic speaker to line out converter. Or you can remove the factory amp unit and connect the amp audio in wires directly to the speaker wires so you can just use the head unit speaker out connections.