Mike,
Now that is a schematic I can live with, old school :-)
So there is no magic here, it is about as expected. The coil of the relay is energized by the thermostat turning on. The relay contacts close and handle the motor load through the fuse from the battery as a supply, so there is no ignition interlock. The only thing missing is a suppression diode across the relay coil. Without that you will get arcing and erosion pitting of the thermostat contacts, as they open, when the relay coil field collapses. That is a likely source of our fails. Should be able to get a relay that fits with that diode protection built in. If the hand notation on the schematic I got from Doug is correct the relay is a Hella 87411 which at least has a suppression resister across the coil which is a help, a diode would be better.
Later Ed