Where does a car head unit draw it's power from?

Question

Is it from the switched feed or the permanent live feed.

Answer 1

BOTH

you need a constant so that when its turned off you don't lose your clock and eq settings, and the switched tells the radio to turn on with the key

Answer 2

A factory radio might be designed either way, but almost all after-market decks will draw the most power over the constant power wire.  The accessory wire just acts as a turn-on switch.  Without voltage on both wires, though, an after-market deck won't work at all.