The wheater was hot.
We back to car after shopping.
I started the car and asked my son stand under the roof.
"Wait the car cool down first."
Does it sound natural?
"Wait the car cool down first."
Is not correct.
Please try: "Wait for the car to cool down first". The addition of "for" sets the condition. It is correct to use "for" when it is followed by a direct object. Additionally we need to add "to" before the word "cool" resulting in "to cool". Here is a link with some helpful information about the ways "to" can be used.