This is required and enforced by US DOT SAFETY Laws if your trailer is equipped with Electric brakes.
You also must have a working 12VDC Battery setup when you are towing your trailer. The DC power to operate your Electric Brakes will come from your working on-board 12VDC battery setup.
Not having all of this working on your trailer in the event of an Emergency Break-away event could go against your Insurance claim and even could result in a fine from US DOT Safety Law Enforcement folks.
Kinda of important in my way of thinking...
If any of what you have needs replaced or repaired etc... look on ETRAILER.com for replacement items...
Roy Ken