If a small portion of the rear window defroster grid is scratched off, you may still be able to use the defroster, but it may not work as effectively. You can try to repair the scratch with a specialized defroster repair kit, which can be purchased at an auto parts store. Alternatively, you can take your car to a professional to have the defroster repaired or replaced. It's important to address the issue because a damaged defroster can impair your visibility while driving in cold or rainy conditions.