The Japanese automakers had a strong end to 2017, but face a challenging 2018. The big seven manufacturers’ exports from Japan climbed 4% in December. Toyota expanded quickest in December with a 19% rise in exports as it scaled back production at overseas factories. Nissan meanwhile saw a 30% drop in exports from Japan, possibly as it retooled production ahead of the new Leaf electric vehicle. Exports by the big seven to the U.S. outpaced the global average with an 11% rise in December, perhaps indicating an expectation of recovering purchases due to the tax reform. The bigger risk in 20...