In the 2019 Cascadia, the main fuse panel is located inside the cab, on the driver’s side.
(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.) 2019 freightliner cascadia trailer light fuse location
If your trailer tail lights and side markers are not working, but your brake lights and turn signals are working: In the 2019 Cascadia, the main fuse panel
If you replace the fuse and it blows immediately the second you plug in the trailer, you have a short in the trailer wiring (likely a pinched wire or corroded pigtail). If the fuse blows without the trailer plugged in, the issue is in your truck's 7-way socket or the chassis harness. (Note: There is also a fuse box under