To completely eliminate all the noise from the cars electrical system, you need to physicaly decouple the power supply from the car by creating an independant pseudo ground. You can do this with a smps, or an isolated power supply unit, or design a discrete isolated power supply. Buying an isolated 12v power supply, then dividing your various voltages is the fastest answer to a noise free supply, but the unit alone will cost atleast $60, but more realistly $80, then you still need to design the rest of the power supply. It is very expensive to do in low volume. My new boards will come stock with an opamp that will run hapily on +/-12v, so i wont need the additional 15v supplies. I do have an isolated adjustable bipolar supply available for hungry opamps that can feed up to 21v, but its not a cheap addition either. I am waiting for my boards to come back from india to do final testing, and then will release my power supplies for use with these boards.
Thanks
Hans