Heavy Duty Wiring Harness:

The common adapter rule is that adapters should only be used to go from bigger to smaller plugs, but there is a wiring harness that can get around that rule. Manufacturers took a 7-Blade Plug and wired a 4 prong onto it so that we could plug into a 4 prong on the back of the tow vehicle. In addition to wiring on the 4 prong, they also wired on 3 extra wires. Now all 7 wires on a 7-Blade plug can be hooked up to power from the vehicle.

Harnesses are useful in upgrading vehicles which already have a 4 prong or 6 pin wired in, but need to use a 7-Blade plug. The harness takes advatage of the wiring work thats already been done, only requiring that the installer splice the extra wires needed. Below in an image of part number 578U that is commonly used when installing a brake control:

There are several versions of the basic harness shown above. New innovations incorporate more and more options into these harnesses. Below is a brief look at the other styles common in the industry.