Circuit to supply power to USB during car brownout
The way to address this is to charge a large value capacitor up to 12V through a diode. The diode is there to prevent discharging the capacitor when the 12V source drops out. Then select a 12V to 5V switching regulator module that is powered off the 12V stored in the capacitor. Make sure to select a switching module (buck regulator) that is specified to work in a wide range of input voltages (for example 7.5V to 18V). Such regulator should be able to supply the output 5V as long as the capacitor voltage is greater than the minimum input voltage of the switching regulator. 

So then the source brownout occurs the capacitor will be able to supply the regulator till it has discharged the capacitor too much. Select the capacitor value (or add additional capacitors in parallel) till your load is maintained through the brownout.