I wonder if a thunder shirt would provide him additional comfort

My dog used to love car rides until we got rear ended one day. After that she was so anxious in the car - like her body would go catatonic and she would shake and pant like crazy. She wouldn't even look at you if you were trying to console her. Just sheer panic.

I, too, was desperate to try anything and this was one of my last resorts. We did all of the classical conditioning to the point where she was fine with getting in the car and driving around UNTIL we would hit a bump in the road or stop suddenly. We also tried CBD, Benadryl etc. I really didn't think a thunder shirt would help, but it stopped the catatonia and shaking!!

She still gets nervous and pants, but it made a significant difference. Combined with Benadryl on our last car ride (3 hours), she almost fell asleep!! I think the constant pressure makes her feel more secure honestly.

My dog is a 65lb lab/pit mix and gets hot sometimes, so I got her the sport one! It's very well made!

We also use a car hammock and buckle her in, but after reading comments on your post I might try crating her to see if it helps.

I use the PetSafe Happy Ride Booster seat for my cat, who tbh is probably larger than your dachshund, and he loves it!