[Gasoline Fumes]

You could use the existing HVAC blower, unless your "2x high-static-pressure 120mm 12v PWM fans from Noctua" are more efficient. Like most (all?) cars, the Insight has cabin air exit vents in the back, inside the rear bumper. They have rubber flaps to keep air (and exhaust) from going in, but open easily to let air out. Bring fresh air in, and the hot air will find its own way out.

For control, I would just have it come on when the interior temp is above whatever temp you want to avoid, with a low-voltage cutoff to protect whatever is powering it. Or if you had enough solar power to run it, you could just let it do its thing anytime there's enough sun.

I have a trio of Insights, so I'll give this more thought. It's something I've always wanted, but never got around to doing it. I'm not sure what is the best air inlet spot if not using the HVAC blower. I wouldn't want the inlet near the fuel tank, engine, or exhaust. The spare tire well is easy and accessible.