I just had the entire interior of my 1969 280SL redone by a company called Heritage Upholstery in Palm Springs California. One of the options that they include in their packages is a heated seat option that includes pads, controls and harnesses. The heating elements are integrated into the seat cushions similar to how a factory install would work, but the controls are basically a remote mounted switch that you place under the bottom of the seats. No need for a secondary pad ontop, totally original looking. I would reach out to Heritage and see if you could source the equipment and whether it would be possible to install them yourself.
I haven't had the chance to reinstall them into the car yet, but looking forward to testing them out.
Peter