Left low beam out on Fisher or Western 3 plug 4 port plow.
This is a common call or email we get: "My plow lights don't work right." or "I just bought new wiring and I think you sent me the wrong kit. I have no low beam on the driver's side".
The truth is it's usually not the wrong kit. It's just that you did not fully read the instructions that came with the harness. This post is referring to 4-port kits and not 3-port. The solution is different, but may get the same symptoms. Read more below.
Below is a diagram showing how to configure the plug:
In the control harness near the isolation module there is a small 3-pin plug (shown below). This plug is called a configuration plug. If you have a 3-port isolation module, this plug is not used at all. It is just left open.
On headlights they now call it a "Common" and common can be positive or negative. (Most truck still use a negative common.) But some vehicles you have to get your test light out to see what way it is so you can configure the plug properly. If the common is negative, the wiring in the plug will be in A & B. If it is a positive common, they will be in B & C. For wiring kits 8437-1 and 27780 they supply a cap for this plug because the wires are not used.
Above is a side view of the configuration plug and below is a view of the pins. 
Please note that the wrong headlight adapters can do the same thing. For instance if you have the adapters from a '98 Chevy in a 2002 Chevy you will have the Left low beam out because the plugs are wired differently.
In the control harness near the isolation module there is a small 3-pin plug (shown below). This plug is called a configuration plug. If you have a 3-port isolation module, this plug is not used at all. It is just left open.
On headlights they now call it a "Common" and common can be positive or negative. (Most truck still use a negative common.) But some vehicles you have to get your test light out to see what way it is so you can configure the plug properly. If the common is negative, the wiring in the plug will be in A & B. If it is a positive common, they will be in B & C. For wiring kits 8437-1 and 27780 they supply a cap for this plug because the wires are not used.
Above is a side view of the configuration plug and below is a view of the pins. Please note that the wrong headlight adapters can do the same thing. For instance if you have the adapters from a '98 Chevy in a 2002 Chevy you will have the Left low beam out because the plugs are wired differently.