Mine got so bad that I took drastic measures:
I used to be in construction, so I took a 3" drill bit (the kind used for making doorknob holes) and drilled 2 new holes in the dash above my stereo. I then cut the plastic housings that feed air into the two center vents, attached new pieces, coated them in silicon and wrapped em in heat-resistant sheets of .4 mil mylar. I had to mount the new vents at a bit of an angle, since my media center is a 430N and takes up a ridiculous amount of usable vent space...but it worked!
It's not perfect...the vents kinda point to the roof, so I clipped a small fan to the overhead speaker bar. Now, when I want heat, I blast it at the roof and the little fan circulates the air down around me. Keeps me toasty in the winter!
Oh yeah. I drive a '12 4-door.