Cars are built with vents designed to always suck some air out the back to assure exhaust does not get in the cabin. Air is always drawn in from the front. Where the windshield wipers are is a gutter, and the fresh air intake is usually under there somewhere. Some higher end models actually have a filter there you have to replace or clean every once in a while.

Incidentally, if you smoke in the car do not use recirculate. As a non-smoker for five years I can smell it. It does not bother me ad I don't get ****y about it but if you go to sell the car it hurts the value and in fact some people simply won't buy it.