RobertISaar: I have chosen Schottky diode so it will be a little less than 0.7V, at 1A it will be about 0.45V (and 0.65 at 5A) but anyway - you are right - there is much to improve, I will consider using ignition signal.
Frank Lee: Pragmatism! That's the spirit.
Old Mechanic: With 4GB SD card it boots for around one minute (sic!) and the thing is that I stop the engine quite often. When I listen to FM radio that it boots up within one second but you know, I live in Germany, would you like to listen to german radio?
So I have to listen to the music from sd card.
ISO7637-2 defines couple of conditions which car's equipment has to withstand. Voltage drop during engine start is simulated with ISO Pulse 4 and as far as I know the voltage can fall down to even 5 Volts or something.