Im currently trying to render a scene in Blender 2.8 with Cycles out of the inside of a car. The side and windows are all basic BSDF Glass Shader. Outside of the car is a cube with a basic BSDF Principled Volume Shader. But whenever I place my camera inside the car, the volume is not visible in viewport as well as in the render.
i assume: your car windows dont have a solid thickness. this leads to the render engine assuming the volumetric is inside the volume of glass (a refractive material) which at the moment is not possible. In your case just add a solidify modifier to your windows, and all be fine. its a faulty modeling setu you try to render, you use a glass shader always get sure that the glass object has thickness (= a front and backside, which clamp the refractive medium).
unfortunately volumetric shaders (like principled volume and volume scatter) dont work inside a refractive material. steps to reproduce: make the standard cube have glass in surface- and principles volumetric in volume shading: it doenst work.