An extension config will only cover up some of the deficiencies, I've now completely reworked the visuals of over 1,000 cars in my collection and I would estimate that 90% at least, needed full extraction and significant material/texture/shader changes and quite often, 3D model changes to get to a reasonable level. So while adding a config will be a decent improvement on some mods, it won't help much with a lot of them.
While this may solve the issue presented, it's really not good general advice... Exterior glass should always have a set fresnel value, I use fresnelC: 0.1 / fresnelEXP: 2.5 / fresnelMaxLevel: 0.6 as otherwise the glass will have no reflective quality and in many cases, won't even be visible. To solve bad exterior glass you need to make sure it has the correct shader type (ksPerPixelReflection), it's set to transparent and that the texture and shader properties are set correctly and alphablended (copy a texture and shader preset from a Kunos car if in doubt). This will solve 95% of cases and in the other 5%, it'll be down to problems in the model itself which then need fixing in your 3D editing program of choice.