Clicking issue is now resolved and I have heat. It was the servomotor that required replacing. Thanks for the reply and for anybody who stumbles on this thread, you don't need to tear apart your dash to get to this motor.
The motor is located on the underside of the heater box in the top left...
more info on this problem and cheap fix
Grinding/clicking under dash - air mix servo - Toyota Nation Forum : Toyota Car and Truck Forums
looks like more than a few 4Runner's have had this Air Mix Servomotor failure
thanks for posting a reply and I think you're bang on with the problem here. I just tore apart the whole dash and took the front cover off the heater box to see exactly what could be blocking the movement and nothing was there. I pulled the motor out of the arm guide and turned the dial to hot...
I have the exact same issue right now and from removing some dash trim I can see a motor at the bottom of the heater box driving various arms. When I turn the heat up, the motor attempts to drive all of the arms attached but gets jammed mid movement. i just don't know what could be jamming it...
