Correct me if I made any mistakes here;
The Webasto's are stand-alone heaters, autonomous systems that run on the fuel in the fuel tank, but do not require the car's engine to run. It has it's own fuel pump that draws only a little energy from the car's battery.
Great for those days when you are stuck in the snow and need to spend the night in the car, waiting to be rescued, haha!
There are models that run on gasoline, and others that run on diesel.
The diesel ones are still popular, and are widely available, new and used.
I know of a brand new diesel system that was on offer on eBay Kleinanzeigen, only 2 months ago, a model that was supposed to be installed in an Unimog. Think it was 300 euros..
Here's today's link for webasto's there:
https://www.ebay-kleinanzeigen.de/s-autoteile-reifen/ersatz_reparaturteile/webasto/k0c223+autoteile_reifen.art_s:ersatz_reparaturteileI have not yet seen a diesel Webasto for W116-specific, but that is probably a coincidence.