If it's an real billboard car, you'd be dense not to.
If it's an impostor then leave them off and focus on the beautiful design on the car. Just my three cents.
The car was originally green. it is painted black now. At this point, I really don't think the fact that it originally had billboards matters much.
If you where trying to put it back to stock, then yeah, you would "need" to put the billboards on. Since you are not, it is just a matter of if you want to.
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