[chiph]

Modern cars have a cabin air outlet. It's usually hidden in the trunk or the back of the cab (on trucks), and vents somewhere like behind the plastic bumper. There are rubber flaps on them to ensure one-way air travel and keep water out. Perhaps Tesla's has a motor on theirs to keep it closed against the increased cabin pressure.

From a 2015 Honda Accord, it's #35: http://imgur.com/QsJexwI

Side-effect of this part - manufacturers tune them so that the doors produce a solid sound when closed.

[darkmighty]

Just by pumping air inside through filters a positive internal pressure is automatically created. How much pressure will be created is proportional to the pump's flow rate and inversely proportional to the car leakage constant ( I think it's something like leakage ~ k * P.I.P., which would make P.I.P. ~ pump flow/k ), but some positive internal pressure is a given even with those outlets.