They use a highly classified raygun like in "Honey I shrunk the kids" to reduce the cars to 1/6th the size, push them in, then use the raygun to reverse the process. Voila!
Although how they got the highly classified raygun in the first place is another story......
Some dealerships who can't get ahold of the raygun use a similar technique as those who build the sailing ships in the bottles.
And others use dehydration. The demo cars are dehydrated, similar to dried fruits, reducing their weight and size. Once inside, you simply add water to reconstitute the cars.
Or they simply use a oversize door that's not readily obvious, and just drive them through.