You can run the cable from the fuse box between the windshield and the dashboard. It stays invisible and it does not affect your air flow. Just take some plastic ruler and tackle it inside.
In case you do it through the vents, with time the cable will collect a kilo of staff and your air quality will be poor. Also your defrosting time will be longer and the air flow funny meaning that part of the windshield will defrost and part will not.

WAY faster!

 

I ran the cable through one of the front panel vents (not windshield vents), and I love it: no cable hanging in front of the radio. Cool !