I think it's a check valve or "anti backfire valve". It's very similar in size and design to the ones used on vacuum brake boosters. The ones on a booster protect the diaphram and the one in the wiper line could stop the violent snap back the wipers do when the engine backfires. The vacuum chamber inside the motor is 4 or 5 times as large as this item so it wouldn't have enough reserve to even make a partial swing.
