The location of the woofer on a door - regardless of a good baffle, enclosure, etc - just the physical location at the front corner of VERY many doors, relative to the dash, seats, console, glass, roof, etc...
The complex reflections from that location to the listening location at your head VERY often results in a common midbass null.
A corvette is a wider, lower-roof car with a more angled windshield and entirely different shape hatch and taller center console, so you may simply be lucky with that car.
And to the OP - I suspect this is the real issue you are facing, although that plot looks like an oddly shallow rolloff rather than a sharp dip like most reflection/cancellation interior/location based nulls...
So the first thing I'd try is replacing the eights (which might be in a "too small" enclosure) with 6.5s that have a low Fs - to eliminate the chance that the cavity is simply too small.
(And if it is - one option for running the eights would be drilling a big hole or three with a hole saw right through the rear of that enclosure, so that they are actually running open baffle)
You don't necessarily need to go to the extreme of running a midbass in an infinite baffle outside your car (although DumDum is and it's impressive - see his build thread), but moving the midbass cabinets under the seats is a strategy to eliminate the midbass nulls by fundamentally changing where the primary sound source is located - therefore changing ALL the reflections and sound paths to your ears.
So is adding additional identical midbass drivers elsewhere in the car - either equal distance to your ears, or delayed to effectively be time aligned.
Or even, you could run your sub up higher, to be that additional midbass driver.
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