4th Gen tail light issues

[Mustang_Rider]

Hey there. 2018 XLT driver side back lights will not work expect when a trailer is connected. However the signal still wont work. The light bulbs are fine, nothing blown. We cannot figure out for the life of us which fuse is for the back lights. The manual doesn't clearly state which it is.
Anyone know? What else other than a fuse could it be?
Thanks

 

[LazSlate]

I would get a trailer test plug to validate no shorts to ground on the trailer. I have owned many trailers and over time the tail lights and wire can corrode, etc. Easier to test the trailer first.