Ok: the simple way is to take the power feed that would be going to the empeg (before it goes through the filter supplied with the empeg), and put a 3A+ diode in the line, with the white band towards the empeg. Next, get some large 25v electrolytic capacitors - 4700uF ones are less than $2 - and connect these between ground and the empeg side of the diode. Try with one, then two, and so on, making sure you put them the right way round (black stripe to negative).

These just help prop the voltage up at the empeg end when the power goes away from the battery feed. The diode stops the charge from the capacitors from trying to recharge the car's battery when the voltage drops :)

Hugo