I have a 2017, 5.0, RCSB with towing package. I am getting ready for a trip to MN and was running through my checklist. I noticed the charging circuit in the 7 pin was not hot. Pulled all the connectors apart to make sure there was no water or corrosion, I was in MN over Christmas and the roads are a slushy mess that time of year. Everything was good and I was momentarily baffled. In 2015 or 16 Ford had an idea that actually was pretty impressive for a US manufacturer. They moved this circuit in to a control unit. Which one I do not know and do not care to know. Once there is a trailer recognized this circuit is turned on and turns off when the trailer is disconnected. So for the rest of you do not go chasing a ghost. I already have and it is a ghost. Plug your trailer in and check if the battery voltage increases. This is a very low amperage charge rate so do not expect it to charge a flat battery. It is only there to keep your battery alive so the break away switch has enough battery to apply the brakes in the event of a hitch failure.

 

Good to know.