I think my way is much better than using Photoshop, plus it fully 3D. Yes, it's a couple of steps, but well worth it as far as I'm concerned.
So what I do is make sure the Cosmos proxy model is set to Full Mesh (Visualization Method) and then convert the model to a mesh (hit C on the keyboard). Sadly, the model comes in as one single mesh, but they do provide Selection Tags. In this car model, it happens to be the first one. Just double-click them to see what is what since they are not labeled at all.
Now with the car body selected, right-click and hit Split. This will make a new object with just the body. It will be confusing at first since the the new body object will also show all of the materials and tags as the original. The biggest place to make a mistake is not realizing the original model retains those polygons. Just delete them and now, in this case, you have the original car without body panels and the body panels are a separate object. This second object can now be used in the Include section of the Decals object. I would also recommend cleaning up the new object by deleting all those extra tags and materials. It can get messy.
In searching for this technique online just to make sure I didn't forget a step, I found a tool in C4D that I had no idea existed. Funny how not reading most of the manual kept that from me. Ha. Anyway, the guys over at PixelLab posted a quick tip about the Polygon Islands to Object tool. Wow. In the right situation, that could be a life saver. However, with the Cosmos models, it can create more work. I used this tool on this same car model. I perfectly created separate objects of every part of the model, which is great. BUT, ever part is called polygon something and EVERY part has ALL of the tags and materials from the entire model. Usually, you can right-click on the body paint material and select Select Material Tags/Objects and all the parts with that material will be selected so you can at least group them together. Works better with other models I'm sure, but nice to know it's there.