Next to those fuses, you will find a labeled "TRAILER LIGHT" or "TLM" (often position R12 or R14 ). If the relay fails, fuses won’t matter. Swap it with an identical relay (like the horn relay) to test.
If you've determined that the trailer light fuse is blown, here's how to replace it: 2019 freightliner cascadia trailer light fuse location
For 2019 and newer Cascadias, individual trailer light circuits (marker, turn, stop) often . Instead, they use electronic relays within the SAM Chassis that automatically reset once a short circuit is cleared and the ignition is cycled. Next to those fuses, you will find a