2023 FRHT Cascadia. Left turn signal blinking fast and not working when connected to a trailer.

That sounds like a ground issue.

 

I HAD AN OLDER TRUCK DO THAT IT WAS A BAD CONNECTION ON THE ALTERNATOR WIRE

 

You may want to do some voltage drop testing on the ground path to the sSam. Path is battery to Mega Ground Junction Block (MGJB) to outside ground frontwall pass-through-stud to the Inside Ground Frontwall Pass through stud to the sSam. After, testing you may find that cleaning & tightening the connections points where you have the biggest voltage drop beneficial. I have seen some trucks where the red enamel cable sealer was put on cables ends before final assembly, rather than after final assembly and was creating high resistance. Please update us with your results.