1986 Jaguar XJS Replace windshield wiper motor

The windshield wiper motor on my 1986 XJS went out 3 years ago. I have had two different, very experienced American car mechanics attempt to install no less than 6 used wiper motors - all were said to be good working motors, in my car. None of them have worked when installed in the vehicle. The wiper switch seems to work - it has 3 wires for different speeds, lights up the test light at all three different speeds. Most recently the mechanic discovered in an old repair manual a diagram that showed an "earth strap" as a ground. Even with the earth strap installed per the manual diagram, the motor still won't work once it is installed. What are we missing or doing wrong?
What is the secret to replacing the wiper motor in this vehicle?