Sounds like the switch is bad

I have a 95 grand marquis and i had the same problem. I fixed mine myself by replacing the wiper motor. The problem was the motor will still work on high but not low. Its an enternal issue on the motor that wont allow it to fully rotate when shut off and its not accepting the low voltage to run on low. My answer is 100% accurate

It may not be a motor problem.
The wiper motor has 2 windings, fast and slow.
The wiper switch sends a ground signal to each motor winding.
There's a problem on the relay, or at the switch or in the cable to the motor.
Hope this helps.
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