The state’s top highway official says that all 25,000 light fixtures in the Big Dig tunnels must be replaced because of a faulty design or manufacturing defect that led to widespread corrosion and one 110-pound light fixture that came crashing down onto the roadway last year. The $54 million project, expected to begin in 2013, will bring up to two years of headaches for motorists who will face frequent lane closures in the tunnels.