My portable compressor's standard cigarette plug won't fit my bike outlet. I am splicing my portable compressor wires and putting a universal terminal connector at the end. Both my compressor wires are insulated with black and one of them has white lettering. In the electrical world, which one is the positive and which is the negative. I tried both ways just for a couple of seconds and the compressor does work. However, I want to wire it correctly to avoid any overheat/burn the compressor. Again, both wires to the compressor is black and one has white lettering. Which one is positive?