For me, I start off with using the grilles or vent fixtures and experimenting with various scales and rotations. These I make sure on the bottom layer. After I have a general shape I like I then have a few select headlight, tail light, and signal fixtures I prefer. For modern vehicles, I start with the DRLs and then move on to the High/low beams, and signals lights. These (as seen from my Hypercar) consist of a number scaled down fixtures stacked together so it appears to be one from a distance.
This is time consuming and (the devs know I’ve mentioned this SEVERAL times) is depending on the camera angle, I’m always end up clicking on the wrong fixture (some from the other side of the vehicle) which end up moving it out of place. The undo function (control+z) some times works, others it ends up rotating several existing fixtures making it worse off so I tend to avoid using the undo function. I’ve suggested a lock function for fixtures on the steam forums which Rob said they are doing something similar in a future update.