This could also be the shitty tail light sockets that degrade. I have fixed a set and rotate it in across both X5's. By now, sockets in all three sets have been repaired with new solder.

My 1st indication that the problem has returned is that when fully turned to full lock in the tail light socket, the upper light will not burn and I get a check tail light message. If I then back the bulb by the very slightest, the bulb will burn and the error goes away. That's my signal that the metal ring at the socket in the tail is ready for repair.

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