Yeah, it doesn't have to be super hipoly. Even a basic box will work, but only the space inside the box will stay dry, so I just recommended cloning the interior so that the boundaries between the wet and the dry would match as close as possible and you wouldn't end up with some parts of the interior staying dry while other parts get wet/dirty.
Make a mesh that roughly (or exactly) matches the interior of the vehicle, and ensure that said mesh's normals face INWARD. Name it "_cockpit" (the game engine will not render this mesh) and when you're in the cab view, anything inside the mesh stays clean and dry (as shown in my first screenshot above). @Forces suggested a 2nd mesh (duplicate of my original _cockpit mesh) but with the normals facing OUTWARDS, but that causes my windows to be rendered opaque instead of transparent, so I was unable to see whether there was water inside the cabin from outside view. Might try making the 2nd _cockpit mesh transparent and see if that works.
Added a _cockpit mesh to my 2003 TrailBlazer this morning, but looking for any input as this is my first implementation of it. My intention was to keep water and stuff from getting inside the cabin, and as you can see in the first screenshot it does work fine when using the interior/cockpit view. Is there a way to prevent water intrusion from the EXTERIOR views?
Sorry @Forces for the self-plug there, I promise it's the only time I'll do it.