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A big problem is the passenger compartment has either better or worce PLD numbers because of the geometry of the car. Usually one side has more room and is farther to the near side speaker than the other side.

Andy from AF was the one that recommendation on the 250hz Apf and it works . Get a relative polarity track and go try it. ITs the width of the car that dictates it.
250hz is 4ms long. 250hz allpass is a quadrature at 1ms. 1ms delay is roughly enough difference to get things to center up. Divide the with of car into 2 and with of each head rest into 2 than add the difference. That’s should be close to what’s needed to get left and right phase to mirror each other using the width of car in 90deg/270deg shift with the allpass applied to only one speaker (assuming your speakers are in the doors ) you can calculate the exact allass it will be very close to 250hz.

The easiest way to calculate the allpass is what is your path length difference in milliseconds?

Times that by 4 and that’s the frequency in milliseconds you put the allpass on one side speaker. Convert milliseconds to hz to find the frequency in hz
Q should be between.7-1.4
 
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