Thanks, its not so much that I need to limit the current- I could use the 3.3v supply for example or even an aRef 1.1v with a very high resistor. I've experimented with these previously, even though the aRef is not intended for supply it still worked okay
Its more about optimization and control than simply current, 5mA is not a big deal the circuit doesn't run when the vehicle is 'OFF'. I don't want to add the complexity of a bunch of additional mosfets and zeners without a substantial return for the complexity.