To determine your amperage drawn from the cars power source,
1, first determine the 4-ohm rated RMS power output of the total system's amplifier (s)
2, Add these together
3, Multiply the total RMS rating (left and right channels) by 2. This is to compensate for most amplifiers' 50% efficency
4, The resulting figure is the total input power 'drawn' in watts.
5, Divide the wattage drawn by 13 (the average vehicle supply voltage