There are just too many issues to resolve if you use a single mic. No matter where they sit your passengers are all facing forward. So if you put the mic on the center of the dashboard no one will be speaking directly into the mic; those in the back seat will be quite far away, and have the seats, the driver and front seat passenger between them and the mic. If you move the mic towards the back of the car (we'll ignore how/where you mount it...) you'll pick up more of the folks in the back seat but less of those in the front who will be much more off-mic unless they turn their heads - something dangerous if the drive turns his head to speak.
And none of those issue take into account that the vehicle is actually moving. Now add in the engine noise, tire noise, wind noise (even with the windows closed) and noise created by the vehicles around you. You're going end up with faint, boxy, off-axis voices with all sorts of extraneous noise.