All grounds are via the car sheet metal.
Because the wiper motor mounts in three rubber grommets with a brass liner to protect the grommet from the screw threads there is a single small short flat woven bare copper metal grounding strap that connects the car sheet metal to the isolated electrically wiper motor by way of the mounting screw.
By the way Donnie I know you have your doctorate in engineering, and I am only a lowly holder of a BS in engineering. I am not trying to be condescending in my posts but I am instead attempting to help those who are too bashful to post or ask questions not only what hooks up to what but how things work so that they might be able to puzzle out a solution to a problem. (Personally I have found that understanding the theory first helps me to solve the application problems more easily, though nothing really helps in solving the math part of engineering application problems other than a lot of replication).