Secondary is operated by the "depression chamber" (Mitsu speak), the sort of flying saucer looking device at the back of the carb on the passenger side. It is operated by a short piece of vacuum hose that exits the carb close by. Usually the membrane inside the "depression chamber" breaks, and the device won't hold a vacuum, and won't actuate the secondary. Test it with a peice of clean vac hose by mouth. If you can keep pulling air through the depression chamber, it's toast. If it hold a vac, check the line that goes to it for leaks.
Check my website (link in sig below) for the "Vacuum hose diagrams" pages, where all these parts are shown.
Good luck!