So I dont know how youre shooting but I would recommend Manual mode for something like this.
When I light paint I obviously leave shutter open for 10 sec or so.
When I do headlights pass I make it about 250th of a sec or faster. To make sure everything is dark but headlights.
I paint one thing at a time. Tire, Mirror, Headlight, Bodyline, etc...
Then in Photoshop you can use a blend layer "Lighten" or "Screen" and that will take your light paint and blend it with the layer below. Thats how you would have good control.
Then make alphas for layers and brush in the parts you want to show and what not to show.
Hope that helps.
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