I thought this might be useful for someone. I measured the response of the subwoofer in an enclosure outside (backyard, no near walls etc), then measured again when the enclosure was installed in the car. No EQ or LPF/HPF were used.
Dividing the two measurements should give an indication of the response the car adds to the response of the subwoofer. The mic was placed in the driver's position. Calibrated sound cards (response of amp included) were used as well, so the results should not be too far off from reality.
JL Audio 10W6v3 D4 in parallel. Sealed ~0.8 cu ft. Here's the cabin response. We can see that there is quite a bit of boost below 40Hz. From that we might deduce that a higher tuned cabinet (38Hz and above) might work well in this car, assuming that we do not care about sub 25Hz content.
The subwoofer in the enclosure outside:
The response in the car:
Comparing outside to in cabin: