This isn't as smart an idea as it sounds. All vehicle exterior lighting functions, even the ones that aren't mandatory—like central brake lights on cars made before 1986—are highly specified as to their design, construction, durability, numerous aspects of performance (how much light they distribute through a wide range of vertical and horizontal angles, etc). It's far more detailed than just "Well, it's a red light facing rearward that comes on when I step on the brake, so it's a brake light" or "Well, the eBay ad said brake light, so that's what it is". We do not get to just declare any ol' light into whatever function we want. If you want to put on a central brake light, put on a (real) central brake light.
