If they don't want to move, they don't.
They're just all nein, niet, nao, nope, no way Jose. They ain't moving, no matter what you--a mere slip of a human being with a lot less hair--say or do.
This happened with our big Newfoundland girl, Zora, the other day when I brought her home from the vet. She just sat in the back of the car and threw me bored glances when I invited her to get out.
"Look," I finally told her. "Your people jump out of helicopters over the Atlantic Ocean to rescue drowning people all the time. And YOU won't even jump out of my damn car."
She's heard this so many times that she a) no longer feels shamed by me or b) laughs at my joke. She just sits there in the car.
Finally, after about 20 minutes of cajoling and bribing and sweet-talking and threatening, I said to my son, "What is the only thing in this world that makes Zora move when she doesn't want to move." And my son said, "THE VACUUM CLEANER!"
So he went into the house, dragged out our vacuum cleaner, and showed it to Zora. The sight of my son holding a vacuum cleaner there on the street was enough to make her scramble (sort of) (scrambling is hard when you're the size of Sasquatch) to her paws and get out of the car.
So. If you ever have trouble getting enormous dogs out of your car, I pass this tip along to you, totally free of charge.