You're currently running some approximation of an "infinite baffle". The distance from the front of your speakers to the back is fairly long and circuitous, and the back wave will be substantially attenuated before it makes it around to combine with the front wave. This is especially true given the size of the dash speakers. Small speakers just can't push that much air. The maker of your planar speakers sells a small cup that fits on the back of the speaker. You can try that, but I suspect it will not change anything for the reason above. If you want to test this. pull the speakers out of the dash, and listen to them. The put them back inthe dish, and listen again. I suspect you will notice a significant improvement in dash vs out.
All of this enclosure business really mostly applies to lower frequency and larger speakers (lower and larger go hand in hand). I use 12 inch sub woofers in large sealed cabinets in my home system. This speaker sit behind my planners ones and fire toward the corners of the room, so in a sense the room walls create a huge horn. The bass is impressive, but that would be totally impractical in a car.
I suspect the best setup is to put larger speakers in the rear (parcel shelf or in the rear seat bulkhead), moderately sized speakers in the kick panels, and tweeter/mid range speakers as we have in the dash. The tweeters are highly directional, and neither of the two options for mounting (dash, or @rsporsche's clever ashtray solution) are ideal. In dash will destroy imaging (since it is all form a single point), and the ash tray mount will be so separate that the imaging will be confused (one ear will hear one thing and the other another).
In the end, it is a car. Sound is sort of an afterthought, and making it perfect is really really hard.
