From what I understand, and please someone correct me if I'm wrong, but the relay regulates the power though the wires. The upgraded harness with the relay lets you wire directly to the battery and the relay regulates the power going through the new wires. The only part of the existing wireing that is used is one original headlight plug and it is ONLY used as an on/off signal. All current is pulled directly from the battery through the new wires to the relay then to the HID kit.