12v wallwart with a relay to a thermistor (which itself is powered by that wallwart) if the fan itself doesn't have one for on/off. Otherwise just 12v to a wallwart if the fan's own thermistor can turn it on/off. If you only want it on when your amp is on, then 12v wallwart to a relay and the relay controlled by the trigger connector. I wouldn't power it from the relay voltage directly.

Or... If you don't want the (fan's) wallwart sucking vampire current, then connect that to a power strip which has a relay circuit built-in. I'm sure there should be something like that around. I know there are all kinds of power controllers for HT that have this ability - at least I saw them at CES a few years back.