The key is getting the cable up to the required temperature for the solder you are using. Hold the iron against the cable until you can touch the solder to the cable and it melts. As the others have said, you have to use the right solder and flux. BTW, I am not sure that a 45 watt iron is powerful enough to heat up the end of the cable sufficiently before the heat is sucked away by the rest of the cable.