Ford did their "coated glass" electric defrosters, front and rear, on the '74-era T-birds and Marks. It was about $700.00 for the pair, evidenced by a gold tint to the tinted glass. No wires, just the coating.
As for an additional rear window defogger, find one from a '70-'78 Camaro. A one piece blower that fits into the MIDDLE of the rear deck. Just need a fan switch on the instrument panel. As it's OEM, it'll look "factory" where ever it is put. I ordered one on my '77 Camaro and it works well. Helped by heater air when it gets warm A self-contained unit, intake is on the top, output on the other side of the bezel. "Paint to match"..
The current electric rear window defrosters don't get that hot. Just enough to warm things for evaporation into the ambient air.
You might try some glass treatment to make the moisture on the rear glass bead up and run off. Rather than just "stay there".
Just some thoughts,
CBODY67