If the wipers stop with the switch turned on that would be one issue. If they don't park when the switch is turned off that is a different issue.

A proper ground (that brass ground strap) is very important. The motor does funky stuff without it. I spent HOURS chasing problems all because one of the wires is a "ground" and I figured "a ground is a ground". Not so, one is isolated from the other. You need a good ground to the casting for things to work properly.