The passenger front is where all of the heater hoses (mine has both front and rear heat and AC so if your does not it may be different) come together along with the vacuum valves . May also be the bottom radiator hose or possible the bypass hose. Defiantly get a light and track it down. If it has been leaking for a while you can trace the stains back to the source. Mine leaked down the back of the block. GM