You might try sitting in the car with everything off except for perhaps the radio. See if you become nauseated after half an hour. If you don’t, it is probably not the chemical composition of the interior. If you do, then it is most likely the parts of the interior outgassing. If you do not become nauseated, repeat the experiment with the car out in the open, engine running and the HVAC bringing in fresh air. If you do suffer the ear pressure and the nausea, then something in the way the car functions is causing the ear pressure.
I suspect the problem is the outgassing of the seats or some other part of the interior is causing your problem, particularly since you have no problems in an airplane. Since your ears and throat are connected, anyhing that affects the throat and lungs and affect the ears.
If the problem is chemical, perhaps a detail shop may be able to thoroughly clean the seats. My dog is sitting here besides me and he suggests that you might do what he does and ride with your nose out the window.