Slight info correction here guys
Most car speakers are designed for off axis listening, due to the way cars are built. This is not saying being on axis is a bad thing but since the path lenghts are so vastly different means on axis is generally going to focus more energy on one side of your head than the other. Car audio competitors figured several ways around these issues many years ago, to bad the car manf do not hire some of us to help them get it right. Some of the newer high end audio systems from the factory are better but none are really correct.
1) kick panel mids and tweets, just not do-able in many cars and a bit tricky to get it right, overall best results can be obtained this way. Start with just the mids, have them as far back and low as possible, you want to equalize the path lenghts as much as possible. Us a very good vocal CD that you know has the primary vocalist in the center of the sound stage(easier to find this with a good home system or headphones) Normally I aim the mids 5" in front of and 1.5" down from the center of the primary listeners ear opening. I aim the speakers at the side windows, the idea is to reflect the opposite speaker into the listeners ear, may sound bizare but it works, exceptionally well and I would be glad to demo this to anyone interested
Then the tweeter, generally mounted low and closer to the listener, may be aimed the same or sometimes slightly up and forward to get it to couple with the mid better. I port the rear of the sealed kick panels(sealed before I port them
into the lower A pillars and stuff a bit of Dacron into the pillars. I cut a hole in the back of the kick panels to correspond to the opening all cars have into the pillars there, if not able to match them up I may cut and smooth out a hole into the metal to make this work. Some like to port into the carpet and it does have a good effect but not nearly as good as into the pillars, remember, car speakers are not meant for sealed boxes(another generality but almost always true except many subs of course)
2) Pillar mounted tweeters, this is a relatively simple and very effective way to get great center imaging and a high sound stage. Best to sink the tweeters as flush as you can into the pillars to widen the stage, mount them a couple of inches above the highest point on the dash and generally wired out of phase to help time align them to the mids. The midbasses generally work well in the doors, just deaden them well and mount them solidly.
I am not an audio snob or elitist, I have owned the best gear ever, competed at high levels, etc, but most of what I know I am passing on knowledge I have gained from alot of study, hard work, rework, more rework but mainly from associations with some of the worlds most successfull audio competitors and installers. World titles under their belts means they really have put thier hearts into what they have acheived, it is not easy to win at that level. So, If I say something works, please believe me, not doing this for recognition, I want to help you get the most you can from your efforts and expenses pursuing higher end car audio, my reward is in making you happy and saving money if possible as well
For those that do not know, I am in the car audio business but it is far more my passion(as is performance and racing) than business to me and always will be.
Have a great week!