[JRTJH]

All Ford trucks equipped with a "factory installed 7 pin connector" have an ignition controlled relay that will not allow the tow vehicle to supply power to the battery charge line in the 7 pin connector unless the ignition switch is turned ON. This was true with my 1987, 1989, 1993, 1995, 2005, 2010, 2013 and 2015 Ford trucks. Some other manufacturers have this feature, some trucks don't.

For me, it's a great thing because it prevents the possibility of a dead truck battery after stopping for an evening. It was even more critical when the trailer had a 12 volt refrigerator or someone left lights on in the trailer (back when they were all incandescent). Back when (before Ford made the change) parking for a couple hours and not realizing that the refrigerator was on or that a kid (or forgetful self) left the bathroom light on, could mean finding someone with jumper cables before you could get back on the road.