Start with a dollar-store solar garden light.
Twist the housing off to expose the LED and diffuser.
Twist off the diffuser to expose the LED. Inside the battery compartment on this one you'll see a AAA NiMH 300Ma. Some models have a large capacitor instead. Put the battery cover back on.
Sometimes they twist apart, or you may have to remove a few small screws to get inside. You can see the 2 wires leading from the solar cell on top going to the board. I'll snip the LED (checking polarity with a charged battery and a multimeter) and use those leads for later hook up.
Again, at the dollar store I found a AA battery-powered emergency cellphone charger with a variety of tips including the mini USB used by my Crackberry. Cracked it open to remove a small PC board that basically transforms 1.5V into something over 5V, enough to give a little charge to my phone. Sorry 'bout the fuzzy shot, I was drunk when I took it.
I used a little piece of double-sided tape (hot glue gun would work just as well) to stick the charger PC board into the solar light housing and soldered the 2 leads from the light to the corresponding leads on the charger.
Put the housing back together and if you can, coil up the excess wire to the plug. I added an old rubber foot with self adhesive tape I found in my parts bin. It keeps it from sliding around on my dashboard
Voila! It sits on my dash, charges the AAA battery during the day and when it gets dark, it automatically sends power to the charger and then to my phone. I keep my phone in the glovebox for emergencies and this way it's always charged. And all for under 3 smackers.