I agree with @MorganS about the phone charger and just hardwire it. no need for a long cable ???
If you want use a buck converter get one that outputs 7 volts and run it thru the arduino regulator for a little extra input protection.
For the brake switch sensing instead of a voltage divider use an optoisolator like this: H11L1 Optoisolator
cheap, total circuit protection and automatically handles switch debouncing. Protect your Arduino - use an Opto Isolator!