Thank you, Crutschow. That helps a lot. (There is a diode in there somewhere and maybe a pull up resistor but I think I can find another circuit diagram to tell me where to put them.)
I am also interested in improving efficiency of the system by matching the fan consumption to the available heat in the panel. This would require setting the fan voltage higher when the difference between the panel and room air temperature is higher and visa versa when the difference is less. This would require removing the relay at the least and using a power transistor with output in the range of the fan (12VDC 5A, maybe 1A). As I understand it, the comparator is basically a difference switch more than a difference amplifier. Could it be replaced by an op amp circuit that produces an output that reflects the difference of the two input voltages thereby driving the power transistor to vary the fan voltage? (I think you can tell I'm not versed in this technology.)