Harry.

In my case, and I beleive the same with Adam, our webasto's fire up just fine, but in about 1 minute they are shut down by the overheat thermostat. The reason is the circulation pump is not moving the fluid (either at all, or too slowly). The pump is working, as you can hear it fine, so it's not a power issue.

With the Marathon (at least with mine), there are two paths for the coolant to return through the loop. The "normal" path is through a normal-open solenoid, while a normally-closed solenoid blocks the fluid from going through the engine. When you turn on the block heat, these two solenoid valves are reversed, so the normally-open is now closed, and water flows through the engine, and out through the other solenoid valve, which is open.

In my case, that valve works, and I get flow, and heat, although it is slower to heat because there is more coolant fluid involved.

If you look at the picture of Jim Shoen's setup the valve on the right side is the normally-open regular return valve. The one on the left is the return that comes through the engine.

Ray