2011 Cascadia rear tail light problem

The rear tail light on the driver side won't turn on when I have the parking lights on, but the signal and flasher and braking part of the light works. I picked up new one thinking it was bad light but new one does the same, I tried switching the passenger side connection to the driver side and everything works as it should, I know I should have done that before I picked up the new one, lol. What is the best way to diagnose where the disconnect is for the parking light function to work? Also I picked up an new air filter for this truck, anyone direct me to install instructions or video?
lmk
thanks for any help
DD-15 engine

 

Hey thanks for the replay, I have the maintenance manual already but says I need a workshop manual that shows how to install the air filter, it a large retangular filter housed in black outer foam.
I have no fault codes that that I can see, like I said, the light signals, flashes and when the brake light function all work, it's just not lit up like the other one when I have the parking lights on.

 

Its just the tractor light, you have connection that goes right into the light itself then each light have a grey type harness connection which is where each signal seems to be separated, so the signal that tells the light to come on when parking light is selected is not getting to the light.

 

I hardly bob tail and I don't know if they would pull me over for being out when the signal portion and braking and flashers function work on that light.

 

Yeh I guess I have to take it to a shop.
thanks
Matt