Is your intent to circulate air within the car, or exhaust hot air from the car?
The fans that commonly plug into ciggy plugs draw 0.7 to 1.3 amps. These would require a solar panel from~ 10 to ~25 watts. That is a small panel, but not as small as you might imagine. These brushed motor fans will not start turning on their own until they see a good portion of the amperage required.
Until that threshhold is reached, heat will be building up in the motor.
Now your idea removes your vehicle battery from the equation, I assume you are worried about running it dead. Your fan would only theoretically run when the sun is out heating things up.
There are solar powered fans already available that just fit in the window.
http://www.sourcingmap.com/black-solar-power-car-vent-auto-cooling-cooler-fan-p-23246.html
I doubt these really move much air.
The Fans used in computers are typically 12 volts, are brushless, and more efficient and fairly quiet. They vary greatly in CFM, Noise and amperage draw.
I have bought and experimented with many. I stopped experimenting after I found this GEM.
http://www.amazon.com/dp/B000BWEIFU?tag=dollies-20
It is 12 volts DC, variable speed and moves up to 110 cfm. On low it draws under 0.1 amps, on high it draws just over 0.4 amps. It comes with 2 blade guards.
If you have a single healthy, fully charged group 24 engine battery, It could run this fan on high, for ~105 hours before drawing the battery to 50%.
I have 2 of these fans. One exhausting air from my roof. I am pushing air through a marine type mushroom vent. I also have one drawing in outside air in a side window. With both of these on high, It will keep my interior at or below ambient temps until late afternoon when it starts cooling down. I also have silver window shades in all the windows making this possible. It is always strange stepping into the Van on a hot day, and it is not blisteringly hot inside.
Since these type of fans take much less amperage to start, and run, they will work nicely with a small solar panel, But I think you are better off using the solar panel to charge the battery, and using the battery to power the fan.
If you are driving the vehicle daily, you wouldn't even need the solar.
eepwall: