The 0v for the 12V D.C system( NOT to be confused with mains earth, which is not connected to the 12V 0V) is in the EDU .You will be able to see several white or white with either a brown of blue stripe wires , these are also 0V wires out the various parts of the electrical looms and hence the various 12V items.The ultimate 0v connection is the battery negative post connector.