General Motors (GM.NYSE) has agreed to a joint venture with a Chinese partner to build a third commercial vehicle factory in Southwest China – an attempts to hold on to its position as the largest auto maker by volume in the country, The Wall Street Journal reported. Construction on the US$1 billion plant will start next year in Chongqing, with phase one opening in 2015. After completion, the facility will be able to produce 400,000 vehicles and engines annually, although the model it will manufacture has not been disclosed. The plant will help GM reach its goal of producing 2 million vehicles a year in China by the end of 2015. The auto maker already has factories in Guangxi and Shandong provinces.