'79 300D

OK, I purchased a ZeroStart coolant heater, but am unsure where I hook into the coolant system. Below are pics of the unit I purchased and the area where I'm guessing I need to hook in. I think I've got where it comes out of the block and into the heater but I don't see a real good place to come out of the heater and back into the block. Or do I have this backwards?

Should I pickup before the thermostat and go into the block???

See attached pics. Thanks in advance and any references to posted pics of units already installed greatly welcomed.

Bill

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