There is a really old feral female cat at my barn, that has been there since I got there 7 yrs ago. She used to always be out in the fields, and avoided people until about 2 yrs ago when she started coming into the barn to eat. At first she'd wait until I left to eat, then she would eat while I was there, but would run if I spoke to her, then I could "sometimes" touch her back when she ate, and most recently, I can occasionally scratch her neck for just a minute or so before she moves off.
The barn owner recently asked for the cats to be fed across the irrigation ditch, down a dirt rd, due to skunks. So its quite a walk for the old kitty who is deaf, and arthritic, and moves really slowly. But I drive past the barn, and she sees my car and will slowly make her way over, if she isn't already there when I get there at feeding time. Tonight, I needed to stop at the barn to get the pet carrier to try to trap a feral female kitten to get her spayed. I had bought a rotisserie chicken to lure her.
When I got to the barn, the old feral female was there. I felt bad making her walk all the way, so I offered her a little bite of chicken. She took it, so I tried offering her one with my hand in the car. She put her paws on the floorboard of the driver's side. So I upped the ante, and offered it on the driver's seat. She hopped up there. So I got in and sat beside her expecting her to panic with me being that close. She didn't so I gave her some more chicken, put the car in gear and drove off. She moved onto my lap, and sat there looking out the window with a somewhat bemused look on her face, like "Well, I'll be! This is AMAZING!"
Then I got worried that as soon as I stopped and let her out, she be freaked out and run off and not eat. But, no...she calmly climbed out of the car and waited for her plate of food and more chicken. I was shocked that she let me give her a ride, and that she seemed so calm about it. So maybe she now will get chauffeured rides from the barn back to her feeding spot!
Unfortunately feral kitty can't come home with me as my husband is deathly allergic to cats (can't even be in a house where one lives...his throat swells up and he can't breathe/eyes swell shut), plus I have a previously feral dog that survived by eating cats...He'd think I was bringing him take out food).
At 16 my cat learned to ride in the car and absolutely loved it. She paid no attention to any human in the car, sat in the back and moved from one window to the other. She loved, loved, loved it.
I will caution you, it was not a good idea to let her approach the car. She lost all her fears of the car, and would not run from them if headed towards her. As she became more deaf that was not good.
I would park the car away from her, turn off the engine (so she doesn't associate running engine with "safe") and pick her up and walk to car.
You want her to keep her fear/avoidance of cars least she gets run over by someone else who expects cats to get out of the way.