where is a\c drain
Have you checked for any leaks along the door seem on that side or up around the windshield? May not be engine related may just be a bad seal someplace.
The drain in on the firewall (under the hood) on the lower right. I should be toward the bottom of the black plastic heater case. A little tough to get to. Be careful using a piece of wire or a stick to clear the drain - only go far enough to clear it. You may want to approach it from under the car. It will look like a black plastic nipple.
How can you tell if it's working properly?
Looking for a Used Grand Marquis in your area?
CarGurus has 1,800 nationwide Grand Marquis listings starting at $500.
Search Mercury Grand Marquis Questions
Mercury Grand Marquis Experts