resonant

I am working on building a new system using a Audioque HDC3 and a Hifonics XX Hercules. I have heard that each car has its own tuning frequency and that if you tune the box to this frequency it will be louder. Does anyone know how to find the resonant frequeny of a car?

i have a resonant engineering Mt 15 and have the opportunity to get rid of it and get a like new dd9515 G but can't decide on my own will it be worth doing all that work changing over and making new build. thanks!