These were a rarely-chosen factory option. I have them in my W116 300SD for the front and rear seats.
This is what the wire harness looks like.
You can use generic seat heater pads and then solder on the Mercedes pins and plastic, which are common two pin connectors.
The problem is going to be coming up with the connectors that fit onto the switches themselves, and then finding or properly making a wire harness.
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1967 W110 Universal Wagon, Euro, Turbo Diesel, Tail Fins, 4 Speed Manual Column Shift, A/C
1980 W116 300SD Turbo Diesel, DB479 Walnut Brown, Sunroof, Highly Optioned, 350,000+ Miles