For it to work, you need a static environment - in this case, a non-moving car. No one is going to voluntarily sit in a car and let their killer run a hose from the exhaust pipe to the interior.
So you have to create a situation where the car is immobile and your victim is trapped inside for some reason - such as stuck in a snow drift. However in almost any such situation, your victim will be able to escape the murder attempt simply by getting out of the car.
You also have to have your bad guy do some prep work to get the CO to the interior - damaging the exhaust pipe by putting holes in it under the interior near naturally occurring holes in the floorboard, etc. or making holes for it to leak in through.
I've been thinking about this since you posted it and the only reasonably feasible way to pull it off is to drug your victim first, place him or her in the car inside a garage, run the hose to the interior, start the car, and get away fast.