Test your engine bay thoroughly before you cook anything in it.
The writers of the car engine bay-cooking instant classic, Manifold Destiny (available in both printed and Kindle formats via Amazon), advise giving your engine bay the finger -- after you’ve warmed it up by taking it around the block, of course. Get your engine bay hot, then pull over and turn the engine off. Prop the hood open and start touching flat surfaces to see what’s hot. If you need to pull your finger away immediately, that spot is hot enough to cook your food. If you plan to cook under your hood often, you may want to take notes on relative temperatures for later reference. Places that are simply warm (but not hot) might be good for heating up some dinner rolls or anything that doesn’t require a lot of heat.
Exhaust manifolds are reliably hot, regardless of the age of your car. Engine blocks on older cars are usually hotter than newer engine blocks. Avoid anything covered in plastic -- plastic just doesn’t get hot enough.