I'm kind of getting worried that my aux. fan hasn't been on for a while. Even after driving in traffic, or on the highway, when I get home and check I see that it is off. So now I'm wondering if the thing even works. What can I do to bring the temperature up high enough to a level I know should make the fan start?

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1999 Mercedes-Benz S600, 103K miles - garage queen
1988 Mercedes-Benz 560SEL, 89K miles - daily driver
2007 Hyundai Sonata Limited, 31K - daily driver