I would not tap into the blue wire at ext reg, this is for the field winding excitation of the alt. Adding another parallel path would greatly affect the charge function of the alt.
What you could do since there is not any IGN switched 12 volts at the rad support, is to wire the relay coil to the buss bar on the horn relay, 12 volts from the battery at all times, the other side of the relay to the thermostat, then the other side of the thermostat to ground. This would allow the thermostat to control the fan duty cycle, as in running with the key off. Then wire the relay contacts to the same buss bar to power the fan(s). Be sure to use fuses on both leads.