I would recommend a CO meter that can measure in ppm- this is a more complex and expensive device that a mere CO detector.
I would ask if you've made sure that the grills aren't going to catch anything on fire in the car, the stuff in upholstery and seat padding is probably flammable to some degree. Also be prepared for it to be very hot in the car, especially if you have multiple grills.
My experience with charcoal CO was not successful, obviously since I'm posting here lol. I was seriously trying to catch the bus, by which I mean it was an attempt and not a gesture. I did it in my bathroom, I put a CO detector in the room, I filled the tub with cool water and brought the grill in once the smoke had stopped, and let it release gas into the room. I heard my CO detector go off, entered and closed the door climbed into the tub. After five to ten minutes I started to feel really nauseous, and I panicked and exited the room and slept for a long time
I think from what I've read, CO is quickly lethal and makes you go unconscious at very very high levels. But at moderate levels, like the environment I created in my bathroom, while still lethal, CO at these levels does not make you go unconscious so rapidly, and it can cause the nausea and discomfort that I experienced.
If I had known about a CO measuring meter, and if I had confirmation that the gas had built up to super lethal levels, perhaps my attempt would have succeeded.
So I would suggest a CO meter to measure the levels of the gas that's collecting in your car, and I would enter the car at a time when the CO concentration has reached these amounts.
Once the CO gets high enough and you open the door to enter, the levels will temporarily drop as you break the seal, but they will build back up as that charcoal continues to cook.
Again, take this advice with a grain of salt, as it only reflects what I learned after my own failed attempt.
Good luck to you