Harmonica

You may need to go to Harbor Freight and buy a cheap volt meter. They are less than 10. Meter the wires. The meter will show + or -. If it shows - reverse the meter probes and read again. That will indicate which is positive and which is negative. I wire my back up camera to the park lights. I did this because the rear of my 89 Dolphin is totally blind in the rear. I drive with the park lights on so that the camera is on. That way I have a video rear view mirror all the time when I am driving.