You were holding it with your hand from the backseat?
I ask because all these things can have an effect. Your hand will cause the response to be different than it would look if you wedged it between the headrest, or simply laid the phone on the seat where the headrest was.
Now what exactly these changes can cause is hard to say. But my point is that people are giving you input when they don't know the setup and that can lead to bad advice (I'm not knocking you guys, but I am being real).
So, again, I'm asking so everyone can get a clearer picture of what is going on when you take these measurements and hopefully give you more accurate feedback.
truth.
even then, it's still a single measurement point and can vary pretty greatly, especially above beaming (OOOOOOO the 2015 buzz word!), if you move it an inch. who knows how accurate the model-specific microphone calibration is from device to device.
but it's really not much different with a real rta or even the fancy multi-mic wizardry we've seen as of late.
which is largely why i still just use an rta as a tool to work out the obvious response issues, or to help identify specific frequencies i'm having issues trying to locate by ear...while i'm sitting in the car with it on the hood in front of me, heh. call me old fashioned, but body and head shadowing play a huge role in the perceived response.
so getting back to the op's desire to work with what he has...and adding on to where i think erin's going... move the phone around, slowly, and watch what it's doing from the left side of the headrest to the right, up and down both horizontal, vertical, and tipped up/down... watch for clues of the average response around your head... identify anything that isn't fitting your desired curve consistently, and work those out.
is your parametric eq selectable frequency center or a fixed point?
you can definitely pull off a reasonable curve and image with very basic stuff. it won't be laser competition grade imaging, but you should be able to get a convincing stage with just basic head unit time alignment controls and level adjustment via the balance setting. you don't *have* to have separate 31-band eq for left and right.
