Sounds like a happy girl! If this has been going on for a long time, I'm guessing it will take nearly as long to break her of the habit and may depend upon how rigid you want to be with her. If she's roaming freely in the back seat and your SUV is large enough, you could put a crate in the back and have her ride in the crate. This has the advantage also of making the ride safer for her in general in case of an accident and she's less able to look out and see things that excite her to bark at. Some people are able to train their dogs to stop barking in general, like if the doorbell rings or they're looking out the window at home or barking in the yard. You could try training her not to bark in less exciting arenas and try to then step it up to not barking in the car. I haven't had this problem so I'm just thinking out loud.
Here's an article or 2.
https://dogtime.com/dog-health/gener...ebelbecker-faq
https://www.dogster.com/lifestyle/as...ing-in-the-car