Do not waste time looking under the dash or on the passenger side. For the 2019 model year, Freightliner consolidated most trailer circuits to the engine bay PDM.
The 7-way trailer plug on the back of the Cascadia is exposed to road salt, rain, and dirt. Open the plastic spring door on the trailer cord receptacle. Look for green crusties (copper oxide). Corrosion causes a short that will blow your or F31/F32 (Turn signals) instantly.
(Note: There is also a fuse box under the hood/bunk for ABS and power lines, but trailer lighting circuits are almost exclusively routed through the dash panel.)
Pull the panel marked "pull" in front of the passenger seat. Check for any blown fuses related to "Trailer Power" or the "SAM" modules. Passenger-Side Firewall (Under Hood): Open the red PDM box. Look for