The 5v line is just a switch. I'm not particularly worried about it, honestly, but I am curious how to pick a capacitor value to clean up any inducted signals.
The 12v line is my concern. What I mean is it comes off the battery, but while the car is running, the alternator and many other things on the vehicle make the signal very dirty. If I run this signal through, say a inductive sensor, and then divide it and feed it into an input pin, I'll need to either clean up the 12V signal, or clean up the split (< 5v) signal. I hope this makes sense.