I have never seen the 15 second flash, but have read about it. I believe it is the BMW electrical system checking voltages, but am not sure. I equated it with the LED trailer lights, but perhaps not so given surgeondress' post.

I have rented a lot of U-Haul trailers. I have had the fast blink on the turn signals many times, but it isn't consistent even when towing the same trailer, it comes and goes. I found that a separate ground wire to the trailer (so that you are not relying on the hitch receiver for the ground) fixed it completely. That ground wire clipped on to the vehicle, separate from the 4-wire connection. I equate the fast blink with a high resistance from a poor ground. I am very familiar with buying HD flashers for North American vehicles, but found that with a good ground connection, I could pull a trailer with lots of lights without the turn signals blinking fast. I would at least try a separate ground wire before changing the flasher.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue