Technically, you'd need to make sure that:
-you have the right power. Does it take 12v UNregulated? How much amp/watts of power does it DRAW?
-is it stable @ 4ohms? as probably your carspeakers are 4ohm. If it's a 2 channel amp, you could always hook up your front and rear speakers in series (but keep left/right separated, duh!), which makes it 8ohm, at which the amp would probably be stable.