My daughter's dog has car anxiety as well. (Also thunder, fireworks, gun fire, any loud noise of that sort). She'd do an entire 5 hour ride from their home to ours, standing up in the back seat, braced for impact, panting, whining and drooling. We got her a Thundershirt to wear, which helped but did not make her entirely calm. She'd lie down and be hyper vigilant rather than standing up the whole way. I think for a short while they tried a sedative as well for long trips (acepromazine). They did the Thundershirt, did lots of short trips (not all to the vet), and over time she became somewhat desensitized to the experience.
She's 7 years old now and can ride reasonably calmly without the shirt and without sedatives. If they're packing the car for a trip and leave the door open, you'll find her sitting in the driver's seat, ready to go. She's not the type to sit in the car, smiling happily with her head out the window and the wind blowing through her fur, but she's not the basket case she used to be.
I don't remember if you asked for advice on even getting into the car in a prior thread but clearly that's an issue now, huh? What kind of harnesses are you using that she can get out of? Does she ever wear the harness around without getting into the car? If she associates a harness with a car ride, maybe you'll have to desensitize her to the harness itself, wear it for meals, wear it for walks, wear it while you play getting into and out of the car like it's part of an agility course- both doors open, in one side and out the other, lots of high value treats- almost like getting accustomed to a crate. Break it down into its most elemental steps- harness alone, harness plus getting into and out of the car, harness plus getting attached to the seatbelt then getting back out, harness plus seatbelt plus close the car doors and get out, etc. Feed meals in the stationary car while wearing the harness. It seems like it could be a very long process though.
I'm sorry it sounds like you're back to square (minus)-one. Some people have tried Rescue Remedy to help calm their dogs, the Thundershirt, prescription sedatives. Maybe the vet would prescribe a mild sedative you could use as you begin the desensitization process again. As things improve, hopefully you'd be able to cut back on, then stop the sedative. I only suggest them to maybe make the desensitization process go a little faster, hopefully not as a long-term fix.