The second postulate says all inertial frames measure all light wave to be moving at c, no
inertial observer will ever measure a light wave to be moving at c+v or c-v. It might help to point out that
addition of velocities doesn't work the same way in SR as it does in Newtonian physics. Also, keep in mind that each observer is using their own system of rulers and clocks to measure distance over time for a light wave, and each observer says that other observer's rulers are shrunk and that their clocks are slowed-down and out-of-sync (I gave a little numerical example of how these effects come together to ensure they each measure the same speed of c for a light wave in post #5 of
this thread)
