Sounds like she's got herself into a right state, unfortunately this has now become habit and can be very difficult to break. She could have started this because she was frightened or excited by being in the car, the strange movement and things whizzing past. If she doesn't already have one get her used to a crate firstly indoors then move it to the car. You want to try putting her in the crate in the car for a few minutes preferably with a cover over it so she can only see to the front. Do not talk to her, do not make a fuss. The key thing is to stay calm and keep her calm she associates the car with getting stressed and over stimulated you need to break that association. Once she's happy spending longer and longer periods doing that you then need to start the engine. The instant she starts barking turn it off and walk away. You need to repeat this again and again increasing the time you can leave the engine running for a few minutes. Then actually go out for a drive to the end of the road and back, then round the block etc etc. It will take a lot of patience and won't happen overnight.