I have a 2000, short bed that I wanted to install a new 7 way female receiver on. After I remove the old one I realized that it has 8 wires running to the old OEM receiver. What is the 8th wire for? New after market 7 way only has 7 wire spots, as expected, which confuses me. Also, when I cut the old wires, one was "live", I didn't expect that, maybe naively. I got some sparks, what do I need to check and should that have happened? Thanks!