Heidi spent part of her Friday rolling through the heart of Hollywood while watching a bikini-clad model take a shower ... on the back of a flatbed truck. Just another day in La La Land, right?

The scene was being shot for "Germany's Next Topmodel," which Heidi judges and also executive produces. She taped something similar last year when she had models posing on a bed in the same area.

Based on her nonchalant reaction to the good, clean, wet fun ... we'd guess she's producing and not judging. Yet..