The phase shift only occurs on the left side midbass driver, and it's a function of many things IMO. It occurs outside of the crossover region, in the passband of the driver. The biggest part of the problem is that it's in a car door.
The driver is about 75 degrees off axis from my ear, so as frequency increases and the driver becomes more directional, the reflected sound (path: driver's door mounted speaker => passenger side window => drivers side window => ear) becomes predominant. My leg is also smack dab in front of the driver, so it absorbs and diffuses and generally makes a mess of things. The driver's side door glass and windshield also contribute something, it's probably too complicated to really define as the contours of the car are so complex. The result though is a phase shift of approximately 180 degrees occurring over a band from about 100 hz to 200 hz:
So far though I haven't found a way to shift the phase of the delayed signals back in time. Perhaps a DeLorean? Instead, I took the approach of altering the phase curve of the right side driver via an all-pass filter in order to match the phase curve of the left side.
Then added in an all-pass to the left side to pull the phase curve down above 400 hz. With that approach I could get the phase curves of each side to line up relatively well from 20 to 600 hz:
This was all done using MFBP second order all-pass circuits, and powered by a cute little premade dual voltage supply. I made the measurements with REW, then simulated the all-pass filters in REW, and then built and tested the filters.
The end result was very good. Still not perfect, but things like bass guitar lines were much more consistent throughout the frequency range and more centered on the stage.
The problems though:
1) My measurement method of measuring at one ear then the other probably had some error in it.
2) I found that the phase curve of the drivers tend to change with volume. It seems to follow a few defined paths though.
So my plan now is to play with an all-pass that can be adjusted while seated in the drivers seat and see what else I can learn from that.