Windshield wiper switch question

I have a 64 single speed switch and a 65 variable replacement.
On the 65 the switch is arcing (hanging freely to test) when I test it so does that mean the ground to the switch is through the attachment nut in dash (or is something else grounding/shorting it).
wiper motor still turns the linkage and switch seems to work properly, just want to make sure before buttoning it up that it is functioning correctly.

Thanks for the advice

Ed