The dual coaxial wires are used for the FM diversify in the original radio. Try to connect the adapter to the other coaxial lead and see if it improves the FM reception. You should not try to combine both of the coaxial leads together since it will cause multi-path problem (drifting). The main purpose of FM diverify technology is to dynamically switch to the antenna that provides the stronger signal. I believe one of the antenna is the rear defroster itself, the other may be combined with the cellular phone antenna.