The rats aren’t eating anything in your engine bay, and probably not in your garage. Leave the car out one night and put out poisoned food some distance from the house. Maybe the rate will go for the food and die outside. The poison I’m thinking of is the blood thinner that will cause it to bleed to death. It will stay outside to drink water, so don’t have any water available in the garage. Note that this poison will hur any animal. If you have cats or dogs that can get to the poison, you will want to consider that when you place the poison.