removing and replacing the washer fluid resivoir.

Try a penetrating oil like WD-40. The Chrysler parts department has something similar called MP-50. You can get other good products at hardware stores and auto parts stores such as PB Blaster. For really stubborn rusted nuts and bolts use Chrysler's "Rust Penetrant". It will do in a few minutes what WD-40 will do in a weekend.

To prevent the plug from being hard to remove next time you can coat the terminals with some dielectric grease. That will prevent them from corroding too.