Like others have said it could be as simple as oxidation on the contacts in the switch. If you get adventurous I suggest you try using a product called "deoxit" which is an excellent contact cleaner. Though I don't recommend you just spray it into the switch from the outside that might do it, I'd probably open it up and do a more surgical strike for best results.



It worked wonders on my radio's volume control which is another known glitch. I used to turn the volume knob and not know at all what would happen (turn it down the volume would go up and vice-versa).
I was able to spray it in by removing the knob, work it around and have it do its magic and restored about 95% of the function which I felt was acceptable considering how little effort and cost it took to perform the job. It's been over a year and still working fine.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz