the guy said you need to have two flat parallel sides of a room, any sides i.e. floor and ceiling, walls etc and if you play the right freq you can make it resonant.

now..i've looked around for equations and examples and stuff but i keep getting a really high freq for an application like a car.
i was thinking of creating a standing wave across the car, i.e. from side window to side window, length of around 1.3m.
came up with something stupid like 120hz!

are there any equations to find lower harmonics of this resonance freq, i.e. if i play half etc.

And yeah, that equation is right. But that's for a sustained sine wave, which music is not.