How do I make the glow of this vehicle brake light to radiate out from the center along a gradient from black to red, instead of having the whole thing light up?
can you pls show us an image how it should look like? because radiate out from the center is (!) spherical. So i am not sure how you mean that…
1- Duplicate the glass shape.
2- Shrink the duplicate with the shrink/fatten tool.
3- Give the duplicate a strong, emissive yellow material.
4- Give the red glass material a high transmission roughness and make sure it uses refraction. You might have to go to the render settings and increase the “max roughness” in the screen space reflections tab.
5- Use bloom for better effect.
eevee_red_light.blend (986.2 KB)
Hi there 
Also a little thought on the realism of this. If we suppose, there is a light, or led inside, one or more, they may be round or oval in shape, behind this cover.
So, you may also put one or more little emissive “bulbs” inside, so then the light will mot strictly follow the shape of the cover, but should have a more realistic coverage with falloffs and all that
You may also play with giving some texture to the cover as well
Happy blending!
Cheers!
