[Autophile]

Regarding the placement of speakers :

If you have freedom of placing the drivers apart from using the better looking stock locations. You can tailor a much better sound.

Tweeters needs to as far as possible, away from reflective surface as tweeter has a wider radiating angle than a typical cone driver. Due to wider dispersion they induce more secondary reflections which can ruin the soundstange and imaging. Similarly in case of dome midranges, they are even worse as they have larger diameter. Avoid installing both of them on axis as they can beam and ruin your sound. Maintaining equal path lengths is very important.

Cone midranges are difficult to install as they are not enclosed drivers like domes and most of them are bigger in size. They need space behind them to play optimally. Do not mount cone midrange sharing the common volume with midbass.

So in all you have to choose a location which has lesser reflective surface in case of domes thats why a lot of people choose kick panel or A Pillar as they are the only two farthest location in car environment from the listening position. Tweeter should be placed even further than midrange.

Keep the tweeter and midrange closer to each other and if possible in same plane if using speakers with shallow slope (Dynaudio Esotecs) in order to maintain the phase and seamless integration of drivers. You get more freedom regarding placement in case of speakers using steeper slope (Rainbows) because of the steeper rolloff in frequency response of each driver.

For midbass seal and damp the doors completely. Give a solid mounting to midbass and try and bring the midbass as close as possible to the doorpad factory grille to avoid it from firing inside the panel.