Jeez, get a multimeter and set it to DC volts and measure the voltage at the green and white wires. If you connect the green trailer wire to the red lead on the multimeter and the white trailer wire to the black lead of the multimeter and it reads +12V then you know the green is positive and white is negative. If it reads -12V then you know green is negative and white is positive.
As bad as Radio Shack is these days, they still have functioning multimeters for $10 or $20 and it would be a good tool to have for debugging. Frys would be better if you have one near you.