2011 workstar lighting codes

I have 2 lighting codes that I am having a hard time diagnosing. Unit is a 2011` workstar and is a tandem snow plow. The 2 codes are as follows:
1. SPN 878 FMI 5
2. SPN 882 FMI 5
Both codes are activated at the same time and both refer from the information I have gathered to be a fault in the trailer marker light/ license plate light circuit specifically under current or open circuit. This unit is not equipped to pull trailer however the body builders use the trailer circuits to power the dump body and additional light packages. All the lights are working and they are all led lights. No lights have been added or removed. The confusing thing is the code is being set when the lights are turned off off? If I clear the codes with the key on and lights on the codes will not reset. However when I turn the lights off the codes will become active. This is the part that has me confused and makes it difficult to diagnose. Any advice or diagnosing tips are appreciated.

 

Thanks Heavyd. The strange thing is this truck has been in our fleet since new in 2011 and there has been no recent repairs or alterations to the lighting system recently. The truck most likely had the trailer plug when new but usually the body builders use it as there connection point to install there harness and relay control boxes. This code has only become active recently and the fact all lights are working and that the code is setting when lights are shut off has me confused. The unit is down awaiting a particulate filter so when I get the dpf back in I will raise the box inspect problem in more detail.