First off I would like to thank TWAN in the FS19 discord channel for explaining to me how to do this. Without their help, I wouldn't be able to share.
In this video I explain how I make a vehicle decal. The biggest point of the video is to show how to get rid of the infamous "TEXCOORD1" missing attributes. I show how to add a second uv map in blender and assign a specular image.