The lack of grounding is because of cracked solder joints where the harness socket mounts to the wiper motor circuit board. The very first pin is the ground and it's the first one that cracks due to its position. The weight/vibration of the harness hanging off the socket is what slowly kills it.
Super easy and free fix if you have a soldering iron, electronics (NOT plumbing acid core) solder, and even the tiniest amount of soldering skill. Just resolder the 5 joints where the socket attaches to the board. Done. I also like to shoot some contact cleaner on the 3 tabs where it plugs into the motor itself, and into the harness socket, but that's just icing on top - the real issue is cracked solder joints.
Note the very first pin nearest the socket - and how it has a thick trace that goes all around the board. That's the ground. If you look closely at the picture you'll see that the pin has a crack all the way around it where the solder has failed.
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Richard