[paul mora]

There is no need for a ground. The cap makes a electrical contact with the radiator and the with the rest of the truck similar to the ground system on your truck The liquid in the cooling system act as a electrolyte for the current to flow off the anode through the electrolyte to the metal parts in your cooling system (engine block). It then goes through the ground system to the cap and off the anode. This is a complete electrical circuit or corrosion control system protecting the cooling system from internal corrosion by corrosion control. This is can take the place of a coolant that chemically stops corrosion (green stuff)

Distilled water will not make a big difference. The water provides current flow and the the distilled will not effect the the resistance in the water to slow the current flow. You should flush the crap out of your cooling system and replace the coolant