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I have a 2018 F250 (specs in my signature) built on 5/2018 - the truck is completely stock (no added LEDs or anything).
This morning my left blinker started rapidly flashing when turned on. Right blinker and hazards work fine.
I got out of my truck and looked outside and all the turn signal bulbs seem to be working fine - they even blink at a normal rate - so it's just the interior blinker indicator that is flashing fast.
I found this online which seems to describe the problem, but its only for trucks built in late 2016.