I have never mixed in the car, only doing occasional car checks. When Glen said he sometimes finishes a mix in the car, I would guess he mean mainly fine tuning the low end.
If your car stereo system boosts the bass, then that means your mixes will need to be bass heavy in your car, in order to sound full elsewhere. Referencing could help a lot there, which is of course much more tricky in the car. Loudness matching is a big part of proper referencing and that would probably best be done in the DAW, before taking the mix and references to the car. Of course as you do it more and more, you should get used to how it's supposed to sound.