Old topic, but it appears that the thermoswitch in my radiator also gave up. I have an HPE 2.0 i.e., so unlike in the older carburated versions, the thermoswitch in mine also has three wires, as the topic opener describes. I ordered a new old stock one, possibly the last one around, but that doesn't seem to work either. So can I replace my three wired thermoswitch by a standard two wired one? If I keep the collar, as Thotos suggests, I suppose I can just earth it separately... But why does the radiator needs to be earthed anyway, I wonder...?
update: it appeared that a jammed thermostat was the culprit after all. However the NOS-switch did prove faulty as well

Installed a new thermostat and another new switch, and now everything is okay again

That means it's possible that the original switch was working fine, but I did not bother to put that in again as I've waisted enough coolant now

I did try to find a thermoswitch that works with lower temperatures,, but I did not find one.