After you save the squad in the Japanese camp, go into the area filled with trees which has the normal path going to the howitzer. Instead of turning right as usual, go straight up to the top of the area and look for a hidden path. Press Action near the hidden path, and the machete will clear it. Go up the path while clearing more brush with the machete. The secret path will exit just under the howitzer. Note: The howitzer cannot bombard you while you are on this path.