another idea, im not sure how similar the hybrid systems are between the insight and the prius, but prius has a seperate hot tank mounted in the front fender that stores hot coolant for cold starts. Depending on how the insight hybrid system operates, it could be that when the engine stops running and when the hybrid battery is running the drivetrain, there isnt much coolant flow therefore it cools of in the heater core. Just a thought throwing it out there.
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