Three potential causes.

First is a bad bulb, but that is unlikely if it happens on three trailers.

Second is a bad ground to the trailer. I fixed a fast blinking trailer bulb with an additional ground connection, as the hitch ball was not sufficient to provide a decent ground. The fast blinking came and went, but an additional jumper wire with an alligator clip fixed it.

Third is a known problem with LED lights. There are a number of posts about this, and there was eventually an update from BMW to address it. Not sure how successful it was, but there are a string of threads about it, and the SIB may be posted here as well when you search.

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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