Since your application is for car body, please note that during rainfall/ snow the liquid should not form any conductive path to ground. If it happens the buttons will false trigger because of added parasitic capacitance due to conductive path.
Also, for your application you have to design guard in such a way that the guard sensor gets triggered with enough amount of snow/ water (that might form short circuit path to ground).
Coming to to the methods for testing this, you can create an environment with varying temperature around the CapSense button. You can also test with grounded water and grounded snow.