A Mercury Grand Marquis sunroof is designed to manage water, not block it like a windshield. Rain that reaches the perimeter is supposed to drop into the sunroof tray and exit through corner drains that feed tubes down the A-pillars and rear pillars. When those drains slow down or clog, the tray can overflow and water ends up on the headliner, down pillar trim, or in the footwells—making it appear that the glass is leaking when the real issue is drainage. Because Mercury often uses similar cassette-and-drain layouts across platforms (commonly similar on Capri, Cougar, and Marauder), the symptom pattern is predictable: wet carpet, musty odors, and stains near visors, grab handles, or pillar edges. Parking angle matters too; being nose-up can bias overflow toward the front drains and A-pillars. Understanding the water path first prevents unnecessary parts replacement and helps you choose the right fix. In many cases, restoring drain flow and cleaning the tray solves the leak without any glass work. If you start by confirming whether the tray drains quickly at all four corners, you can separate “water management” problems from true glass or bond failures and avoid chasing the wrong repair on your Mercury Grand Marquis.