My solution to the reflections was eventually to try and use them to my advantage. The closer you can get a speaker to the apex where windshield and dash meet, the better you are at mitigating diffraction. At that point you are able to use the windshield more like a wave guide.
There's a specific calculation that will specify where the diffraction break up occurs based on distance to the glass. So I ended up putting my tweeters in the A-pillars and then used PEQ at 8khz to help cancel out the breakup. If I can't hear it I don't care about it. But I'm sure if you measured it'll tell another story. I also messed with 16khz with EQ to help control the reflections too.
Long story short, unless you hit 100% on my volume majority of the reflections are dealt with. The down side is that the shape of our dash and angle of mids ends up skewing the stage a bit. I also think it has to do with whatever trick they used for "driving on center". I can move a little bit in my seat and the stage steers left and is damn near perfect. If I sit comfortably in my seat then the stage is to the right a bit.
The only other way to deal with reflections is to learn about psychoacoustics. If you go that route then you basically want everything to be point source and on axis. Kick panels work very well at mitigating issues with reflections due to psychoacoustics and their distance from glass. It is typically the best when trying to maximize your path length distance too. The down side is installing them so that your habits don't get in the way of the speaker. The same goes for the passenger.
