I'm sorry your daughter is going through this. Panic attacks are very scary and the fear of having another one can also cause one. The fear of fear.
I've suffered and still suffer from panic attacks. I've tried to use logic but you just can't do that while having one.
One of my mind tricks that does work is finding an alternate way of doing/going somewhere. It's not the healthiest approach but does work at times.
If she won't get into the car (for fear) you can always try an alternate way of getting there if possible, walking, ect., at least for now.
Also if she is on a benzo you can have her take one 30 min before getting into the car to help stabalize her. You can remind her she is safe, you are there for her and nothing is going to happen. Reassure her she will be safe..have her talk her way through it while in the car..try to be patient and kind. They are scary when they happen., remember it is VERY real to her even if you can't make sence of it.
If you can get her through the ride to see the doctor please make sure the dr. is aware of whats going on.
I'm sorry she is going though this and you are strugggling to help her.