In your Mercedes.py, you should import the car.py file as follows (as long as the two files are in the same directory):
import car
Then you can do:
Mercedes = car.Car('Mercedes', 'S Class', 'Red') #note the necessary 'car.'
Alternatively, you could do
from car import Car
Mercedes = Car('Mercedes', 'S Class', 'Red') #no need of 'car.' anymore