Have you tried an LeD strip under the rear deck grill. 10-15$ on eBay for 2 strips. ( or you can go to any motorcycle shop and pay 30-40$ for the same thing.... they are used by many motorcycle riders to add brighter brake lights.
i can’t find my old post, but here is another sample of the same install.
My car/picture bellow is from 2017... LED I used were much brighter than the ones used in 2015 sample link attached.... they seem to get brighter every year. Buy the brightest you can find. My tail lights are also LEDs.