It'll take time. Just keep taking her out there and sitting with her with the engine running and without it running. Maybe sleep out there a few nights with her. She just needs to get comfortable with it.
My cats HATE, HATE the van. But, they had no choice with a cross country trip. Not to mention the 2 plane rides they had to endure. They came out fine.
One would sit on her crate and look out the window...she's not too smart at times and Big Boy would hide under the seat. He would come out to eat and use the litterbox, then back under the seat he would go. That was his "safe haven". Once in a while he would venture out to sit on someone's lap for a bit, but he got used to the van. Guess he figured he might as well because there was nothing he could do about it.
Just give kitty some time, she'll get used to it. Don't leave her home, take her with. I'd LOVE to take my cats with me everytime I traveled, but since I do fly alot, it's not possible and there are alot of hotels that don't like cats.