Here are my thoughts so far. Since car battery is 12V and can reach up to around 15v when running, I am planning to power the arduino with a step down buck converter to 5v
Some people like to use an external voltage regulator, but the Arduino has an on-board regulator and it's rated up to 20V through the barrel jack or Vin. (I have an Arduino in a car powered by '12V' through the barrel jack.)
The LEDs can be powered by 12V, through the MOSFETs & usual current limiting resistors. If you try to power all of those LEDs through a 7805 (or the Arduino's built-in regulator) the regulator will probably overheat.
[u]Here is a MOSFET driver circuit[/u]. (You can leave-out the diode, it's for inductive loads like a motor or solenoid.)
The other 5V on pin2, I will use it as an input which will be connected to the brakes positive..
Don't use a voltage regulator for "signal". Voltage regulators are for power supplies. Use a [u]Voltage Divider[/u] (2 resistors) with an added [u]over-voltage protection diode[/u].
I haven't tested this on actual yet. But will this work? And if does, since I will be using pin 3-13 as output, I will then need 11 MOSFETs.
Try it first with just one LED per channel and without the MOSFETs. Then "build-up" the circuit a little at a time.. Add the MOSFET with one LED before adding the others.