Just going through that with my '64. From what I've read, it seems that you won't fry it if it's backwards. I believe it is like a light bulb, as long as power goes through it, it don't matter which way. I tested mine to see if it works (it was out of the truck) by putting wires on it and just touching them on a battery. The needle would jump to H hooked up either way but what I DID notice is that wired one way, the needle would hang on H for a few seconds after I removed the ground wire as if there were some sort of residual power and wired the other way, it would immediately go back to C. I made a note of which was which and will wire it so as the needle returns to C right away.
