The cabin air filter, in your 2004 Mercury grand Marquis, is located behind the dashboard on the passenger side. The air filter is directly behind the glove compartment.
For a 1999 Mercury Grand Marquis : ( the engine air filter is Motorcraft FA-1668 )
99 G/M has no filter.
That car doesn't have a cabin filter.
Mercury Grand Marquis was created in 1983.
The 1997 Mercury Grand Marquis owners manual shows : ( Motorcraft FL-820S ) and fordparts.com shows : ( Motorcraft FL-820 ) for the engine oil filter
Its located on the passenger side before the rear axle in the middle of the rear doors
it is attached to the stater
on the transmission