How do I replace a heater core on a 9991 mercury marquis?
I don't know, the 9991 Grand Marquis has not been built yet, but
if I know FORD, they will put it somewhere where it will be tough
to access similar to the thermostat or the 2nd spark plug from the
rear passenger side. Autozone has a do it yourself feature on the
internet that has a lot of instructions for that kind of stuff;
they will want you to register first.