Electric Rear defrost?

Thanks for all the help but I'm not talking about rear air/defrost my car came with that from the factory and as far as I know so did most of them but I have an electric rear defrost with the lines running through the back window I saw it in my 74 and 75 dart brochures it's at the back if you have the brochure. I have never seen the switch for It and have no clue how to set it up.

Thanks for the info about the spedometer I don't think I will ever use it but it does make a good spare and is very interesting.

Dan