Stray voltage in trailer pigtail

I bought a 2017 Tacoma in August 2017 and had an aftermarket tow package installed through the dealer. Including wiring for the trailer lights. Over this past weekend, I found that I have a 9 volt reading in the pin for the left side trailer light. with the truck turned off! This has NOT been the case since i got the truck, since i had an LED strip plugged in for the past two months.

I traced out the wires and none are rubbed or scraped bare, there are no obvious shorts. I removed the left tail light assembly, where the shop spliced in the trailer light harness wiring, and everything looks normal. I measured the voltages at the pins using a multimeter. The left tail light is at 9v, the other pins less than one. All with the engine/key both on and off. It seems to me that they should all read no voltage when the truck is off and key turned off.

Anyone got any ideas??

 

New development is that, as of today, the pin that was reading 9 v is now reading less than 1 v, just like the other pins. I am wondering if there is a relay that provides power to the tail lights and maybe it is sticking.

 

My trailer light wiring includes a control box that has a wire back to the battery. It provides power to the trailer lights so they are not drawing power from the truck's lights. If you have a similar setup, maybe there is a short in the control box.

 

thanks, but I don't have a control box like that.