Yeah I've been wondering about that too...yet another dream mod for my dream 968.
They've been mandatory in Europe for years. Not only do they look sweet (esp on this car) they're safer; sometimes they cant see your front or rear signals (assuming theyre paying attention at all, that is).
My fathers Volvo 1998 S70 has little "Volvo" plastic caps covering the holes where they would fit, these days most European makers just leave em on for export on this side of the pond.
Our 968s don't have that little plug though...so guess you're looking at some serious bodywork, i.e., a sweet looking and effective mod...but sadly perhaps not affordable for many.