In most US states it is illegal to have two, high-intensity red rear fog lights. Other countries have visability limits on when they can be used. I believe early US (at least) Diablos had only one. I know on my 99 the backup lights could double as fog lights but the lights are physically different including missing a wire from the lights to the main harness.

The thought is that the fog lights would be confused with brake lights and it seems to be a mystery as to why thats a bad thing

-mick