I recently bought a new car and was very surprised to find that it has no rear windscreen wiper. On further investigation, I discovered that in general, car manufacturers only include rear wipers on vehicles they consider hatchbacks, not sedans. The reason I've seen given is that hatchbacks cause a slight vacuum as they drive which attracts more dust to the rear of the car than for a sedan. My last car, a Mazda 323 Astina, looked like a sedan (it had a sloping rear window, not an almost vertical one) but was classified as a hatchback, so it had a rear wiper.
However, when I'm driving my new car and it rains, I lose rear visibility due to water drops on the back window. More water collects on the rear windscreen when stopped at the lights than when moving at high speed but it is a definite safety issue. Every time it rains my hand itches to turn on the non-existent rear windscreen wiper!
Why is rain not considered a problem for rear visibility in sedans?