Headlines declared the auto industry achieved a showroom mini-victory in March by markedly improving sales over a dismal February. But the improved numbers were something like lipstick on a pig as March’s boost nonetheless concealed some ugly realities. One of the most foreboding trends to continue despite the March uptick: the still-accelerating plunge of the full-size pickup truck market. If segment sales do not stabilize this year, revenue-ravaged automakers may have to take drastic measures.