Here's a correction to the correction: D1 isn't a reverse-polarity protection diode. It is a transient undervoltage clipping diode. It clips all negative voltage spikes to -0.7V or so, keeping them from damaging the circuits from negative voltage spikes coming from the alternator on the 12V battery feed. As shown, the circuit in discussion doesn't have a reverse-polarity protection diode. Leave D1 as it is, but add a reverse-polarity diode in series too. Notice that D1 and D2 (the zener) work together. They each clip negative voltages to ~-0.7V, but the 15V Zener also clips positive voltage spikes down to only 15V. I wonder if D1 is needed at all actually, on second thought, since D2 will clip negative voltages down to -0.7V or so by itself anyway. See the "Zener Diode Clipping" section of the link below, for instance.