How rare is electric rear defrost?

How rare is Electric rear defrost? I picked up a 73 Dodge Dart Sport to part out. It has the fold down rear seat and something I've never seen before in one is the rear defrost with the grid in the back glass. The switch is mounted on the bottom of the dash below the headlight switch. How rare is this option and what is it worth?

I've seen the typical blower style rear defrost. I had a 74 Dart Sport with that. But have never seen this style with the electric grid in the glass. Here are some pics. Looking for any help with this.

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