After washing your C5/6, water is trapped behind the taillight lenses, and leaves a dirty trail below the lenses after you drive off.
For an easy solution, I tear off a small section of paper napkin (keep several in the glove box), and wedge it under the lenses while I dry off the rest of the car. The napkin acts as a wick to absorb the trapped water......just remove, and throw away before driving off. BINGO. the area below lenses is clean & dry!