Imagine a box where each side has an area $A$, and suppose that it contains a continuous flow of rain that falls vertically at a velocity $u$. The density of raindrops is $\rho$ (that's the number of drops per unit volume).
The rate at which the drops hit the bottom panel will be $\rho Au$ and the rate they hit the vertical (left) panel will be $\rho Av$, giving a total rate
$$ R=\rho A(u+v) $$
In the case of a car, you have a single windshield of total area $A$ at an angle $\theta$. Its horizontal cross-section is $A\cos\theta$ and its vertical cross-section is $A\sin\theta$. So the total rate will be
$$ R=\rho A(u\cos\theta + v\sin\theta) $$
where $v$ is the speed of your car.