The only bummer about using fiber in cars is that there is only 1 main light element. When is burns-out or fails - the whole car will not have lights. (Would be a pisser if they failed while cruising down a dark road @ high speed.)
I would imagine that they would have to come-up with some sort of back-up system to make it work.
Also I understand that the lighting units are $$ and give off a lot of heat.
But fiber in cars would be cool just the same. (I'm sure we will see it some day soon.)