Lets do it step by step. Sur ewe will find what’s going wrong.
First you check the arduino : You succeed to dim a led (with the fading example of the arduino IDE I supposed). This means your arduino is working well.
Then you must check the motor shield : Suposing you are using the fading example of arduino IDE, means that you actually fade the pin 9.
Plug your shield onto the arduino, power the shield with an external power supply (+ PWR -). (12V-1Amps for example)
Then connect your voltmeter between the output 3 and 4. You should see a 12V fading output ? Is that working ?
Then about the motor, you say that it’s a car carwindow wiper motor, are you sure you motor run with a 6V, 2.25Amps power supply, did you try that ?
Does you motor look like this ?
For the pwm, as colorsound mention, there is 6pwm output on the arduino, and you can control them with v4 firmata or basicmata example patch.
You say you wanted to control 7 motors with v4. No pb with that. But first you have to tell us if you want to control them on both direction or not ?