Well, you'll not go to jail, but underfloor lighting is strictly speaking illegal in germany and only allowed for show cars - and even then it has to be off on the road. (StVZO §49a Absatz 1)

Violating that rule invalidates the permission to drive that car on a public road, immediately voiding insurance coverage. You'd better not have an accident with that car, because you're personally liable for any damage caused. That might not bring you to jail, but can seriously ruin your life if you cause a permanent injury to someone else.