This is a two part tutorial, one part is the text, the other is the video. Make sure to watch and read both!
Create a new material
Set the material the "vehicle_decal"
Copy the body
Delete everything that's not part of the livery
Delete any parts of the livery that wont be seen, stuff like interior trim.
Assign the reflective livery material to the body
Move the reflective body slightly outward in its respective direction, that way its in front of the normal livery.
Attach the reflective body to their respective counterparts
The way it works is you have a copy of the body,(or just the parts you want to reflect, but for compatibility with other skins I suggest doing the main body) overlaying the model. It is set to the "vehicle_decal" material, with a special reflective livery texture.
The reflective texture is the same as the normal,except it only has what you want to reflect. You want to delete everything except for the reflective bits. You also don't want any unique or identifying features on it, examples would be unit numbers, license plates, "supervisor" or "traffic unit" text. Since you can only have one reflective texture unlike the livery, it needs to match up with every one of your normal livery textures.
Example of the normal livery texture and reflective livery texture
It is very important that you select "vehicle_decal" and not "vehicle_decal2", if you select the latter, the reflections wont work nearly as well.
We only want to do the reflective parts on L0 and L1, since anything beyond that is unnecessary and will waste polys.
It is very important when changing the material from the livery to reflective livery to edit the UV mapping and have "copy from channel 2" and "generate new mapping" selected. Otherwise some sections of the body may have bugged mapping.
Make sure to replace the mask texture in the reflective material with the vehicles dirt texture. Otherwise, when the car gets dirty, its going to look weird.
If you are making a specialized and unique model, (like Michigan State Police for example) you may just want to select only the portions of the body where the livery would reflect, saving many polys.
In the video, I rename the parts to either L0 or L1. That is just to keep things organized. It wont have any effect on the model itself.
In this video, I only kept sections of the body that would have the reflective parts. For greater compatibility, you may want to include more, including the door trim and roof.
Results may vary, some colors just seem to reflect better.
Some misc pictures and videos showing the effects.