wipers operate on high only and won't seat in off position
You probably need to replace the wiper motor.
It usually is a relay; start them on high and try to find it (it should click); change it and everything should return to normal.
It should be here
Looking for a Used Grand Marquis in your area?
CarGurus has 3 nationwide Grand Marquis listings and the tools to find you a great deal.
Search Mercury Grand Marquis Questions
Mercury Grand Marquis Experts
Related Models For Sale
Used Cars For Sale