NoobMan: RAMPs has 2 diodes to offer reverse polarity protection on the 5A input (the one that goes to the Mega & the stepper drivers).
D2 is reversed across the input (causing a direct short if the supply is reversed) and D1 is forward biased from the 5A input to the VIN on the Arduino.
D2 is reversed across the input (causing a direct short if the supply is reversed) and D1 is forward biased from the 5A input to the VIN on the Arduino.