Being from Alaska and recently South Dakota I've had an block heater on my 8V71N and in 2000 my 8V92TA.
They are easily installed. but the coolant must be drained.
To test either the cord or the plug just use a multimeter and test for continuity between the prongs. If no continuity the unit is bad.
They should have a plug in box in the engine compartment to allow them to be used without an extension. The box should be wired so that it doesn't get power from the inverter.
They are available on ebay or other sites. I recommend you get the one for your individual engine. If you dont, it probably won't fit. If it doesn't fit correctly. it will fail or short out.
Here's a site with a basic discussion:
https://www.dummies.com/home-garden/car-repair/diesel-engines/using-an-engine-heater-in-a-diesel-engine-for-cold-weather-starts/
