The formation of rust is the consequence of an oxidation reaction. The presence of an anode, cathode, and electrolyte can cause oxidation. The metal in a car can act as both anodes and cathodes and water is an electrolyte. If the conditions are humid enough, oxidation can occur even when the car is isolated from the environment. Once the metal starts to rust, it is very difficult to remove it. So it is recommended to prevent any such formation with Rustproofing methods. There are various ways to achieve rustproofing. They are galvanizing, painting, and passing electricity and you can also use rust proofing spray to avoid rust.

Factors that influence the formation of rust in your cars,

Oxygen and iron have a strong affinity for each other. They seek the other even in hot regions where the air is dry and react with each other. Consider rust as their love child.
Salt has been used to remove the snow and ice formation on the road for many years. It is adopted in various places as it is the cheapest way to deal with the roads in winter. The salt helps to lower the freezing temperature of the water and you can drive your car safely over them. All seems well, but the sad part is the moisture that is present on the road reacting with the body parts of your vehicle, especially the underside.
People living close to the sea have their cars affected by rust all the time. The closer a car is kept near the beach, the more exposure it gets from the combo of salt and water. The exposure is directly proportional to the formation of rust.
The iron has some additives with them that can prevent rusting. The best example of this is stainless steel. It has nickel and chromium that binds the iron atoms together, preventing them to react with oxygen atoms and this keeps them from oxidizing.
A hot iron can rust quicker as the presence of heat can catalyze the chemical reaction. This makes the mufflers and the exhaust manifold in the car more vulnerable to corrosion. Also, thin iron can rust faster than a thick iron.
An iron that is painted with oil or other paintings has no contact with water and air. This can slow the process of rusting.
Poor maintenance is also a reason for the formation of rust. Neglecting a car without any care can equally damage the car as the other things mentioned above.

On the whole, the formation of rust and the speed of its progress depend on the material that is rusting and the surrounding environment.