the sending unit should ground itself via it's mounting screws, otherwise just run a wire from one of those screws to a good ground spot .... might check the wiper motor direct from a battery and see if you got a defect, and it may need a ground wire, if it has rubber mounts

Bill

Moved over to the Passing Lane

"When we tug a single thing in nature, we find it attached to the rest of the world" ~ John Muir
"When we tug a single thing on an old truck, we find it falls off" ~ me
Some TF series details & TF heater pics