Mysterious plug behind driver side headlight

[Ford Cowboy]

On my 2000 f350 I noticed right behind the driver side headlight is a plug that is not hooked up to anything just laying around back there. It has a yellow and orange wire coming out of it and they seem to be tied into the rest of the headlight harness. I put a circuit tester on the plug and it always has constant power comin to it even when the truck is off. I have no idea what it goes to. Any thoughts???

[timmyboy76]

i believe its a plug for the hydraulic control unit (HCU) for the brake system. It does a self check after the key is turned on. Trucks' equipped with the device, it sounds like a growl winding down.