I have no idea how the Flex-a-Lite fan controller works.

By what I read on a quick search, it powers the fan on 60% at the set temperature and goes up 10% for every 10 degree increase in temperature.
I assume it's changing the voltage going to the fan but I could be wrong. Did it change the speed of the HHR fan?

Power it up, connect it to a digital multimeter and put a heat gun on the probe. See what the meter says when it gets hot.

It's possible that these fans only trigger on 12 volts and pulses the power to vary the speed. It's raining right now so I can't test variable voltage on the fans.
If the controller varies the voltage and these fans only cycle on 12 volts, you would need to set the fan controller 40 degrees colder than normal to trigger the fan.
Again, I'm grasping at straws here since I don't have one to play with.

I'll try to figure out how they work but it may take a while. I know the older Mercedes had a stand-alone fan controller but I don't have one to play with.