For bunnies that were once bonded, the car rides can really help. It can take a few weeks though, so you have to be persistent. And I know this can really be a pain. I think one of the best things you can do though is when you get home from the car ride is still spend some time with them in a neutral territory – like a bathroom or even a bathtub. (just no water of course!)
During the time in the neutral space, pet them both, feed them healthy treats – unless they fight over them) and then make their neutral time together pleasant.
This way they can continue to see the other as something positive. 1. Comfort during stress (like card rides) and 2. Yummy treats and affection while they are in a neutral territory.
Make sure to keep things short if they start getting aggressive.
It also may be helpful to start switching their litterboxes, every few days, or even the bunnies themselves.
Another thing you can try is using what the "stunt double" technique – at the bottom of the bonding section. Basically get two toy bunnies, and make sure each smells like one of the bunnies. So Belle would have one, and Ferdinand would have one. (you can leave them in their cages for couple days, rub the toys over the bunnies, even take some of the fur (from a brush) and put it on the bunny. Then once each bunny’s toy bunny has their scent on it, then put Ferdinand’s stunt double in Belle’s cage and vice versa. I know with Jack, he would first hump it, charge it etc, but then he realized it was zero threat and then when he met the "real" bunny Rucy, he was so calm with her. Now this is no scientific thing. Just guesswork that seems to work sometimes.
None of this is in replacement of the car ride, it’s just to help assist the whole process.