On the first mission in the airport (the one with Frank), you can get a car into the main area of the airport.
First of all, take one of the new, rounded cars to the airport as the car you select for this mission. You should get the Schubert Extra-6 at the beginning of this mission anyway, and that works perfectly.
It is important to only do this trick AFTER you have taken out the guys inside of the main building and Frank has run for cover. The car you brought will be parked in the last parking space on your right as you enter the airport for the first time. Make note of it but don't use it until after you've taken care of the guys inside.
On the cement posts opposite the ones that are on the right side of the main building in the airport parking lot (they'll be facing a field), there is a considerable gap between the leftmost post and the building it's next to. It looks wide enough for a car to squeeze through, and it is, but not without a bit of force. It's technically a little too small, but like most physics engines, the one in this game will allow you to force some solid surfaces slightly through each other.