Stubborn trailer - GIF

[hemmjo]

Battery backup is working fine; taillights are still functioning

As I see it;

Tail lights are still functioning because the pigtail is still plugged into the truck. On all of the trailers
I have ever worked on, the battery is for brake activation only. When the trailer finally broke totally
free of the truck, the pigtail pulled out or broke and the lightes went dead. Then brake safety cable
pulled the pin on the trailers emergency brake switch.