It you are using the "de-ice" function, this might be the normal case ... warm dry air must be blown onto the windshield in order to remove the fog and dampness. With just the heater running it will push warm moist air to the same spot which would add more problems to the visibility issues.
Check for a heater control valve in the upper hose to the heater core, they stick all the time, then see if the air mix door is functioning properly on the heater itself.
Just replace them one at a time in the exact same order they are now.
The newer models can be changed by hitting "max ac" and "def" at the same time for a couple of seconds.
it is broken
If your radio is the cd/tape combo, or if it has an RDS button, just press it a couple times to cycle until it tells you to set hours/minutes, then use the select arrow to change the time
Press and hold the clock button at the same time press seek either way to change the hour and tune disc to change the minute
if the rear ac is working then you probably have a damper door that is inoperative. this is in the heater/evaporator box. so if it isn't working then you will be heating and cooling at the same time out of the heater box.
You use a timing light to check the marks on the timing chain/belt.
According to the 2004 Mercury Grand Marquis Owner Guide : ( DOT 3 brake fluid , meeting Ford specification ESA-M6C25-A is recommended ) * if DOT 3 is not available , DOT 4 can be used ( I'm not a mechanic / technician but I don't believe that both types should be in the system at the same time )
That's how they work. One button for one window. They don't go down all at the same time.
No one cares!
There is a heater called the EdenPURE Quartz Infrared Portable Heater that can heat your home and purify the air at the same time. It can heat up to 1000 sq feet of space at a given time. You can check out the company at www.edenpurestore.com.