Have you tried:

from PIL import ImageGrab

worked for me...

The problem originates from your ImageGrab.grab call. Unfortunately, it's not mentioned in the documentation itself, but from the source code, you see, that it's:

ImageGrab.grab(bbox=(left, top, right, bottom))

So, you must calculate right and bottom accordingly. This code works perfectly fine for me:

import cv2
import numpy as np
from PIL import ImageGrab
import pygetwindow

titles = pygetwindow.getAllTitles()

while True:
    cars = pygetwindow.getWindowsWithTitle('car - Google Search - Google Chrome')[0]
    print(cars.left, cars.top, cars.width, cars.height)
    img = ImageGrab.grab(bbox=(int(cars.left),
                               int(cars.top),
                               int(cars.left + cars.width),
                               int(cars.top + cars.height)))
    frame = cv2.cvtColor(np.array(img), cv2.COLOR_RGB2BGR)
    cv2.waitKey(1)
    cv2.imshow('frame', frame)

Please notice, I also corrected your cv2.cvtColor call, such that proper colors are displayed.

----------------------------------------
System information
----------------------------------------
Platform:      Windows-10-10.0.19041-SP0
Python:        3.9.1
PyCharm:       2021.1.3
NumPy:         1.20.3
OpenCV:        4.5.2
Pillow:        8.3.0
pygetwindow:   0.0.9
----------------------------------------

This works for me:

#!/usr/bin/env python3

from PIL import ImageGrab

while True:
   px=ImageGrab.grab().load()
   m=px[613,296]
   print(m)

I think it will be faster if you just grab one pixel though by specifying a bounding box like this so you only grab one pixel:

#!/usr/bin/env python3

from PIL import ImageGrab

while True:
   screen=ImageGrab.grab(bbox=(613,296,614,297))
   px = screen.load()
   m=px[0,0]
   print(m,screen.size)