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Since I took off my car tweets to fix a dented dome, I decided to play with tweeter placement. My tweets are the wonderful LPG 26NA's, neo domes which can play down to 3 kHz with a steep xo. Very flat and low distortion. IMO, subjectively they sound better than the Morel neos I've tried. My door midbasses are Vifa P17WG's. Nice combo I lucked into, in spite of a (temporary, quickie) single R and C crossover. A friend who had expensive a/d/s car speakers was impressed. My main f/r anomaly now is a dip from 2 kHz to 4 kHz, which my coming (proper) xover will cure.

I took some modelling clay and long wires, and picked some test CDs. So I would have a reference for A/B comparisons, I recorded my test cuts onto MiniDisc and used my portable MD player and Grado headphones. Interesting that if you listen for a long time (minutes) your ears get used to f/r anomalies and they become less audible. Whereas if I go from my Grados to my car system, I can within seconds spot f/r anomalies. With the same track playing on my car CD player and on my Grados, I can pause one and switch to the other within a second, and f/r anomalies are plainly audible.

Anyway my original locations were beside the midwoofs forward in the doors. I wanted to see if I could place the tweets on the dash to get better imaging (or its facsimile) without making the tonal balance suffer.

I found out a few things, listed in no particular order.

1. There is a slight effect of "hey it sounds like the mids in the doors and the tweeters are on the dash" but lifting the image up to the dash level is IMO a worthwhile tradeoff.
2. You can't get the singer image in the center of the dash. It can be made to sound like it's in front of you (driver).
3. My criteria was the singer should sound like she's in front of you (Kate Bush in my case) but when the "s" sound is sung there's a tendency for the s to sound like it's coming from 2 separate tweets instead of where the singer is. Annoying IMO, like when your home speakers are badly placed.
4. It is possible for the s to sound coherent in front of you, or at least not sound like it's coming from a tweet on the left and a tweet on the right, but quite difficult.
5. To get good imaging, the angle of the tweets is critical. 10 degrees off and it's gone.
6. The best imaging I got from the drivers seat entailed non-symmetric pointing of the left and right tweeter, and I'd imagine it would sound bad from the passenger seat.
7. All of the best tweeter positions was near the A-pillars. If I do exact cross firing (driver tweet pointed to passenger and vise versa) I get the migrating "s" problem above.
8. The detail (hi treble) is much more pleasant than when my tweets were in the doors pointing to the occupant's knees.

I've narrowed down my choices to two positions. Pointing at the dome light above the center console, and second, pointing at the near occupants chest or something like that.
I'll do the exercise again when I have my active xo's in place - the results might be a little different.

I don't know how to fix the tweeters at such an odd angle.

In general "imaging" in a car is more like a pleasant arc of sound at the dash level, with no real specific palpable images. Is this what the car audio rags call "soundstaging". Pushing the tweets into the farthest corners of the dash pointing up sounded terrible. It sounded like a boxed in tweeter. In general pointing at the glass to reflect off the glass was bad. My LPGs sound better than my home AR's soft domes. And I generally don't like the "sharp metal dome sound".

Disagreements welcome.
Cheers,
Jason