I had the same on my mistral. Sometimes it would not work, but if I held the switch on and tapped the relay box in the door, it worked.
So I took out the relay and levered the plastic cover off the top and slid the electrics out. Ater looking at the solderes on the board I noticed a ring around one of them telling me the solder joint was cracked. Out with my very old soldering iron and a quick dab and all has been well since.
This same thing also worked on my Intermittent wiper relay. I had them sometimes working and sometimes not.....same thing. A quick solder and all good since. :lol