The black wire is power in (probably from the plastic push button switch).
Black to brown is low
Black to green is high
I have a 2587564, NOS MoPar, if you are still looking.
Do you have 2 or 3 speed wipers? A pic of the wiper motor will tell us.
And am I to understand that when you flip the switch to "off" the wiper arms park but don't go down enough? Have they ever gone down all the way since owning the car?
Back when I was a Californian I did quite a bit of "testing" by getting 2 gaskets and putting a metal shim between them. There was even a factory block off plate for use on 49 state applications. This was when we had to pass a visual test by the CHP at roadside inspections.
You might have to...
If you were to ground the feed wire at the sender, turn the key to 'on', does the dash gauge go to full? I used to use a remote starter push button for this as my shop was a one man show.
I rebuild 4-5 of this type per month. You do need to get it apart (the rivets) to get at the corrosion that is between the contact (green corrosion) and the rheostat (spring)
1969 Imperial had 2 different style dimmers.
This style I rebuild
This style I do not
You might want to at least feel up behind it to see if it is even plugged in.