Step 1: Collect Your Materials
1 Solar Light -- These can be bought from Wal*Mart and most home improvement stores. They're the kind that you stick in the ground. They charge during the day and shine a small light during the night. You can buy them in singles or in packs. If you are going to buy in bulk, try ebay, as they will be cheaper. If you are only buying one or two, ebay is not a good option because of the shipping costs.
1 Small (personal) Fan -- These can be found just about anywhere for less than a dollar. I happen to have an old one sitting around, so it was free for me. Ask your friends if they have an old hand-held fan that they don't use anymore before buying a new one.
You'll need something to construct the ducting. I used duct tape and a vacuum attachment, but in another step I'll offer a way to make ducting out of just duct tape.
You may also want a couple of switches and some spare wire, and possibly some weather striping, but it isn't necessary. As with many projects, make sure you have plenty of common sense before starting this project!
The only tools I used were a soldering iron, a pair of scissors, wire stripers, and duct tape.
Step 2: Disect the Solar Light
Remove the circuit and solar panel from the plastic cover. It is probably only held on by 2-4 screws.
Step 3: Desolder the LED and Add the Fan
Next you need to add the fan. Before soldering it in, though, check the polarity to make sure it spins the right way. You'll probably want it to spin the same direction as you would if using it as a normal fan. If you want to save a little time, when you remove the motor from the fan, mark the negative lead so you don't have to check it later. I didn't find it to be a problem, but if you want to save 3.2 seconds, mark it first.
After you get the polarity correct, solder the fan in. Once again, be careful with your soldering. The leads are very close together, so you'll have to be very precise in order to keep it from shorting out. If you find that your leads stick pretty far out from the board, use a pair of nail clippers to trim it close.
At the moment, whenever the solar panel is not receiving enough light (and the battery is connected), the fan will turn on. To keep this from happening, I added a switch between the board and the fan. Also, the only way the fan will turn on is if the panel is covered (and the switch is closed, if it's added). I didn't mind having to cover the panel, but if you like, you may cut the negative (the black one) lead and solder a switch between the panel and the board. Whenever the switch is open, the IC will think that not enough light is hitting it and redirect power from the battery to the fan. Note that with either setup, the battery will not charge while the fan is on.
As mentioned before, the LED only takes a small amount of electricity. For this reason, a resistor is likely placed between the battery and the LED. For most fans, this resistor is unnecessary, so you may want to remove it and add a wire to make the connection. This will give the motor a bit more power.
Also, most fans take 3V via two 1.5V AA's. I found that my light only had one 1.5V AA, but I think that you could add an extra battery in series to the first one. It (in theory) would charge properly (it would take longer), and would give the fan its 3V. In the same place you buy your light you can probably find the rechargeable batteries for it. It would likely be easiest to remove the old battery clip and put in a new, 2 battery clip. I'm not positive this will work, as I was content with the way mine worked on its own.
Step 4: Make Some Ducting
Be sure to attach the solar pannel and battery clip to the cylinder!
Another way to make the ducting is to take duct tape and stretch it out to whatever length you want the duct. Stretch a second piece to the same length and press the very sides of them together. Peel off two more pieces of duct tape to the same length and attach them just two the insides of the other two pieces (leaving just a bit of to the side). Do the same with the other piece on the other side, but don't leave any extra room. Finally, roll the whole thing to connect the pieces in a roll. You may want to add wires between the tape to add support. The description is a bit difficult to understand, so see the pictures for a better idea of what I'm talking about. (Sorry for the lower quality pictures.)
Step 5: Use It
I (unfortunately) shorted mine out when I tried to reconnect the fan, so although I know that the battery will power the fan and blow the air up and out of the ducting, I didn't get the chance to test it in a car. I will soon buy another light to actually test it with, but the commercial I first saw this product on showed that it will keep a car at around 75-80 degrees F, verses the 108 degrees F the control car got to. Of course, it would be just as useful to connect a pair of batteries to any fan, but I wanted to see if I could make a solar powered one for much less than the commercial. I would love to hear about anyone's experience with one of these and any ideas for modifications to the design. Happy hacking.