If it is like my 123 chassis 300D block heater most likely is a 400 watt unit. No fuse, not thermostat. When she is pluged in she is on. They have an electrical connection at the block heater itself (near block) which can get bad. Also according to the people who make new ones for MB Philips and Temro (phone 952 941 9700) they should not be plugged in when car is idling or as is the case driving down the street which I did once (oops!). Unplug before start up which I never did for two years. Heaters when operating right give about an 80 degree rise over ambient air temp. Contact company they will tell you how to test them.

I used to have mine dangling over front bumper. I respliced and have it in the engine compartment area
Tom