It appears that it was installed in the hose to the expansion tank! If so, that simply won't do, as the coolant doesn't normally flow thru that line.
The temp gauge not showing heat is proof it isn't working properly.
Better tell them it isn't going to work, and hopefully they will change it at their expense! I think they should have installed a block heater for that price!

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!