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I would have suspected the blinker control module, but there is also a separate rear combination control unit that may be acting up.
Are there any extra complications such as an aftermarket reverse camera installation? Do all four (six if on the mirrors) of your blinkers work when using your turn signal (front and rear left, then front and rear right)? Did replacing parts create this situation or was that in response to the fast blinking? What happened when the fast blinking started?
If no aftermarket changes, then the FSM is likely where to look for ideas (the dealership is probably using that and should have been able to fix it for you). Here's a link to all the chapters in your FSM: You can use the FWD and IDX chapters to find your way around.