A few have used the low beam head light wire as a switched wire (search for CBDanes' driving lights). I wouldn't use that as the power, only use it as a switch for a relay (too many amps pulled will cause a fault). Run power from the battery to a relay. The headlight power could act as the switch to turn on the relay; then, relay (switched power) to your usb charger.
If life gives you MELONS you may be dyslexic.