Wipers do not work anymore

Some days ago I found out the wipers do not work anymore. Of course during an unexpected rain. That's a bit disappointing because I have checked all the wiring last year as I have renovated the cabin inside and also the wiriung in the engine bay was checked two years ago. Also the wiper motor is a new one.

What I have found out:
1. The wiper motor gets 12V when I put the switch on (slow, fast, interval).
2. When I give external power the the wiper motor contacts it works.
3. I have found here some threads here about missing ground for the wiper motor. Giving ground to the motor body does not help.
4. What also brings the wiper motor to work is switching wipers on, removing the plug at the motor half so that's there still a 12V contact and giving ground to the
three contacts (slow, fast, interval).
5. So it looks like there is a ground problem for all three contacts. I have checked the wiring diagram and there is only a realy, the switch and of course the cables. The wipers seems to get ground at all by the green relay below the dash, right? So should I check if the relay ground connection is OK?