Thanks George!
I'm not exactly an EE, so I'm real green on most of this. I knew that car voltages can be nasty, but as per other discussions around these forums, people have been running Arduinos from cars seemingly fine. The datasheet for the Nano says it should be ok up to 20v. I hadn't even considered reverse voltages at all, does this tend to happen on a car? For reverse voltage, one would throw in a diode on the power line? As for the spikes, a capacitor on the power line?
Honestly the only thing I considered was a drop in voltage on cranking the car. I doubt much that anyone is going to be using the keypad while cranking, so I didn't think a restart would hurt anything. I suppose that the dip could lock up the chip instead of just restarting it. I'll be actually running it in my door soon, so I'll have to see.
If you have any good suggestions for cleaning up the power, or maybe a good place to look, let me know.