Being unfamiliar with the wiper set-up on a Challenger, I don't know if this will help:

I'd start by double checking that the motor is really in park by running it and then observing that the motor arm reverses when switched off. Then I would remove the wiper arms and see if I could then attach the arm by moving the linkage. What I'm trying to say is don't assume that the wipers (if installed) are clocked correctly.

The linkage arm that attaches to the motor arm... is there a plastic bushing at BOTH ends?