You don't want the fan on all the time; even that small draw will drain a battery eventually.
A good way to wire a fan for that use is to use a 30-amp relay. Connect the primary power to the relay to +12 volts, probably at the battery is a best bet. Install a 1/2 to 1 amp fuse in-line at the battery.
Then, use the blue amp turn-on wire from the head unit as a trigger for the relay.
that way, the fan is only blowing when the stereo is powered up. DOn't forget ground connections for the relay and the fan!
If you have a dash-mounted switch to turn your stereo on and off, you can wire the fan power off of that. going this route can prevent you from needing the relay.
Be prepared for the very real possibility that the fan will cause some noise in the stereo system. It doens't always happen, but it does occasionally. If all of the sudden your stereo is buzzing from fan noise after installation try using a 100uF 50v non-polar electrolytic capacitor in parallel with the fan. that has helped on some installs I have been around.
Good luck and let us know!