My 740 does not like rain. Ever since I had a local guy install my new rotors and pads, my brake light stays on. I found a brake pad wear sensor & wire in the trunk. He didn't put in on the right rear. Also when I drive in the rain, after about 15 min. the ABS and DSC lights come on and stay on until the car has been driven for a day in dry weather. If I continue to drive in the rain after the ABS & DSC lights come on, the speedometer quits working and the steering becomes very heavy (lose power assist). I think I have an open connection box where the rear brake pad wear sensor should be connected and when water gets in the connection box it affects the DSC and ABS. I am investigating this now and if I determine the true problem and the fix, I will tell you.
I think your car needs new brake pad wear sensors (2) on left front and back right. Look for ABS/DSC speed sensor wires when you do this. If this comes from having someone change your tires, they could easily have knocked off a brake pad wear sensor. I have always thought that they fit kinda loose in the little slot on the steel back of the brake pad. I hope this solves your problem........ Let me know.