I have seen them. I think it was an aftermarket option, but i'm not positive on that. For whatever reason, it is a great idea since I know that the factory defrost capabilities are pretty lacking! When I picked up my 71 Demon I had to drive it home 65 miles in a heavy rain... I couldn't keep the fog off of the windows! I also learned about leaking firewall seals that night... as the water kept dripping onto my right foot!
I was given a rear window glass from someone cleaning out their garage... it has the electric defrost grids built into it. I had never seen that until the day I got one. The only thing is that I have no idea as to where it gets plugged in to!
