Sounds like it was just wired backwards. If your outlet is like mine, it's got two black wires, instead of a black and a red. Usually, one side will have little ridges running the length of the wire. That side should be positive. You could also check polarity using a multimeter. Positive probe on the inside wall of the socket. Negative probe grounded to the bike. If you've got it backwards, you'll show 12 volts.