What control the wiper speed on a E34 535i 89 ?

I have a strange problem, the first continous speed position now give me an intermittent wiper, so i now have 2 positions with entermittent wiper mode (a bit longer delay with this new intermittent position) and still have the high speed continous mode on the third position as usual.

Any clue as what may be the cause of this and how to fix that problem ?

Thanks Claude