Little and often exposure starting with the lowest level & always partnered with something nice.
For example...
Step 1: Take her out near the car and give her a treat. Nothing more than that. Repeat that several times a day until she shows no fear (even eagerness) from being near the car.
Step 2: Repeat step 1 but with the car boot open - don't try and put her in.
Step 3: repeat but this time pop her in the back seat, keep the door open and sit in there with her. Literally 2-3 seconds in the car, just long enough to have a treat. Repeat until she shows no fear on being popped in the boot with the door open.
Step 4: Give her a treat that takes some chewing as soon as she gets in the back seat and briefly shut the door then open it again. Repeat several times a day (ideally) until she is comfortable with this stage.
Step 5: build up how long you can leave the car door shut for until you get to about 2-3 mins while she eats a treat in the car. Repeat several times a day (ideally) until she is comfortable with this stage.
Step 6: put her in the car, shut the door and start the engine. Give her a great. Repeat several times a day (ideally) until she is comfortable with this stage.
Step 7: put her in the car, shut the door and start the engine then make a very short drive - like just a few metres. Repeat and slowly build up the distance you can drive until you can do 5-10 mins. From then on try and only take her on short journeys for a couple of months or so, so she gets settled in the car. Keep giving her treats when she does well (so long as they don't make her sick) or something she likes to chew, like a kong or a fave toy.
This is desensitisation and counter conditioning and these are scientifically proved methods used by animal behaviourists and trainers to assist any and all animals in getting over fears. There is nothing else except flooding (forcing the dog to spend so long in the car without escape that she has to learn there is nothing to fear) is this is both risky and immoral.