12 week old puppy hates / fears the car

My 12 week old mini Bernedoodle Cooper seems to really hate or fear the car. He's always whined while in the car, but it's turning into real distress anytime I try to take him anywhere (short outings to vet or socialization –20 minutes at most, usually more like 5-10). He whimpers and fights to get out if I hold him in the back seat with someone else driving; and I of course also need to be able to drive him by myself. He outgrew his initial enclosed travel carrier and his current crate is a larger wire one that isn't super practical for the car. I've tried making a nest on the front passenger floorboard with him leashed where he could see me, and he wouldn't settle at all.

I don't think he is carsick; he hasn't vomited or had any accidents at all during or after any drive and there's no drooling. It seems like a fear-based whimper/crying even when car isn't on or moving, and he really wriggles and fights to escape the car itself. He's not a fearful pup normally, just in the car and on outings to unfamiliar places. Today I spent 5-10 minutes just sitting in the back seat next to him, while he was in a harness and new booster seat. I fed him the highest value treat we use but he got so distressed I didn't even start the car. I'm not sure what to do!

Any ideas for getting him better accustomed to the car; or any suggestions for a better set up in the car? Since he responds roughly the same no matter the configuration I'm guessing it's the car itself and not the harness/leash/seat/bed/crate. I have more free time the next 2 weeks and am planning to continue w 5 minutes/day in the car without going anywhere, with high value treats, and work up from there in tiny increments. Or should I try a different approach? Any tips welcome.