Found this.
With your block heater unplugged take a Multimeter and check the resistance on both of the flat pins of your plug. Resistance typically should be right around 10-15ohms. If that checks good your next step is to check the flat pins of your plug with the round ground pin on the bottom. There should be no resistance read from either of your flat pins to ground. If it does check out shorted to ground remove the plug from your block heater and check the two round pins in the heater to engine block. If there is resistance is read the block heater is junk and you need to replace it.
