Fisher / Western Part # 29047 to #26345 / #26346 Vehicle Control Harness is the adapter which is needed to use the 3 port module on a 3 plug plow.
3 plug plows with a 3 port module box need this adapter.
The adapter has a 4 pin plug on one side which plugs into the 3 port light harness,  the other end plugs into the controller harness.