I just bought a new cord from the dodge dealer for ~$20. I have a splice int he original one from an accident and it has been getting water in it a tripping the GFI. It is a cummins part in a cummins box so i would assume that you get it from cummins, fleet pride or the like. I think you can also get the block heater separate or there are other companies that make block heaters for this application that might actually be better.
Ted