If you take a car charger for your vehicle it is possible to buy a female cigarette lighter socket and wire it into an unused accessory fuse in your fuse box. From here, you can run the cable (assuming it is a longer cable) along the driver/passenger side pillars and finally along the roof trim and down to your phone from there. If it is lower on the windshield, then obviously run it along the back of the dashboard. The wire should be very easy to hide.
The most important parts here are going to be
- Dropping 12V down to 5V (which is what the car charger will do for you)
- Fusing the connection on a CLEAN LINE. Adding a load on any factory circuit can cause bad things to happen. The fuse is self explanatory. Safety first!
- Make sure that the fuse that you tie into is an accessory power source meaning that it shuts off with the car. Otherwise the phone charger will have a parasitic drain on your car's battery.
I think this goes without saying also, but just in case, please please please make sure to keep your positives and negatives separate and tape the car charger/cigarette lighter and use proper connection methods when connecting wires (butt connectors / solder).
Hope this helps.