I use my phone as a GPS and I like to charge it while driving around. In my SAAB I used the 12-volt charger and it worked great because the socket in the 9-5 had switched power. It was only on when the car was running.
My Ford Focus has two 12-volt accessory power ports, but both are non-switched power and anything plugged into them runs if the car is on or not. Someday I'm going to wire one of these sockets to switched power. Until that day, I've been using the USB port for the Ford Sync system to charge my phone.
The first issue is my phone sees the Sync as something to connect to and I get a message asking if I want to go into USB device mode every time I plug my phone in. The second issue is the phone doesn't really charge from the Sync USB port. It is getting power, but the GPS program can pull more than the USB can give. So here's the solution to both problems:
Satechi makes a car charger to USB plug. You can find it on Amazon. It's tiny and sits nearly flush with the 12-volt port when it's plugged in. A bonus is it's rated for up to 2.1 amps so it can power just about any USB device out there.
The secret is it comes with a "smart converter" that goes between the USB port and the USB cable. The little dongle allows micro USB phones to connect to computers and USB wall chargers without triggering the "connect as a USB device?" prompt. Instead it enables quick charge.
The USB cable to keep in the car is one I found on Amazon and is a black and red cloth jacketed USB 2.0 to Micro B cable. I think it looks real nice. I keep it plugged into the car all the time (with the smart converter attached) and have routed it under the steering wheel, hiding the excess cable behind the dash.
Satechi 2.1 Amp Car Lighter USB Charger Adapter with Smart Converter
Cloth Jacketed USB 2.0 A Male to Micro B Cable