Terry

You can connect the fan to the set of pins on the receiver which is usually reserved for a separate battery pack – or indeed to any spare set of pins – via a standard servo lead + plug. The pin nearest to the edge of the circuit board is usually the negative rail and the centre pin is positive:

Dave M

Invoking logic, that's how servos get their power. In nearly all radios, the ground and positive pins are cmmoned across the with of the board, so when poer is applied to one pair, it appears on all. The other pin of the set of three carries the control signal for that channel.

If the fan needs to be controlled from a channel, a switcher will be needed, but if it is going to run all the time, since it is only a modest drain, plugging into the radio power will work fine. If the radio is powered via a BEC in the ESC, there will be a modest reduction in run time, if the radio is powered from a separate radio battery, the recuction in run time could be considerable.