The car did not break your mix. It played back the parts your room was hiding from you.
Cause
The car is a second opinion, not a bad speaker
Every room adds its own peaks and dips to what you hear. Mix long enough in one room and you compensate for it without noticing — cutting low end that the room exaggerates, or pushing bass into a dip that only exists at your chair.
A car has completely different problems: a small sealed cabin, seats absorbing high end, road noise masking detail, and speakers with their own low-end shape. It agrees with your room about almost nothing, which is exactly why it is useful.
The parts that change most between the two are usually low end weight, stereo width, and how loud the mix feels relative to commercial tracks.
Check
Find out which decisions moved
Before blaming the car, check three things on the mix you exported: how much energy sits below 100 Hz, how much of the mix survives folding to mono, and how hard the master is being pushed.
Low end that only exists as stereo information collapses in a car with a mono-summed system or a single sub. Width that sounded exciting on nearfields can arrive thin and hollow.
If the mix is heavily limited, the car's noise floor eats the quiet detail and what remains is the loud, flat part — which is why an over-pushed mix often sounds worse in a car than a quieter one.
Fix
Meter the parts a car exposes
Keep low end centered and check mono compatibility before export. Leave headroom rather than pushing loudness to the edge, and reference commercial tracks at matched loudness in the same session.