Check the ohms using a multameter. Believe it should show around 12 ohms. If checking through the cord shows "open", remove the cord and check directly at the block heater. If that shows OK, replace the cord-- [most likely the problem]
I'm no good with electrical stuff, and couldn't hear mine when truck was new to me. So I plugged it in one day when it was cold, waited half an hour, then crawled underneath and felt it....it was warm now I know it works. Kinda ******* I know but it worked for me!!