The Horns themselves are behind the front bumper on the driver's side. I had the horns go slowly from loud to wimpy to nothing. The fuses and horn relay are located under the hood on the passenger side close to the battery. If someone pushes the horn button on the steering wheel and you cannot hear a "click" from the relay, check the 15 amp fuse. If the fuse is ok, then have the relay checked. If you can hear the "click" from the relay, more than likely the corrosion on the horn bracket is preventing a good ground.
Directly behind the steering wheel.
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.
The wiper control module on a 1998 Mercury Grand Marquis can be found behind the instrument panel. It is close to the steering column, on the right side.
In the front of the engine behind the A/C condensor
Behind the intake plenum.
The flasher buitton is on the top center of the steering column behind the steering wheel
On the drivers side of the instrument panel ( it must be somewhere behind the steering wheel )
The heater core is behind the dash the passenger side of the car.
The emergency flashers for the 1997 Mercury Cougar are on top of the steering column behind the wheel.
The stock amp in a 1990 Mercury Grand Marquis is in the dash behind the radio.
In the dashboard behind the small grill to the right of the steering column.
On the left side of the trunk behind the trunk liner ( I assume there is an access hole )