2002 Grand Marquis Mystery

PLEASSEEE HELP ME! My wife and I havea 2002 Grand Marquis GS
Sedan 4D and it is killing my pockets trying to figure out what we can do to
finally fix this on and off going problem. The car will not keep charge for more
than 3 months. We have purchased and charged serveral batteries. When
driving it will suddenly cut the headlights off. The car will still run but we
watch the battery gauge drop suddenly as we are driving. Many times we do
not turn on the radio in fear that the car may cut off. We have repleaced the
starter, battery, thermostat, and replaced and cleaned the catalyctic
converter. We are constantly told many things but it doesnt stop the problem
from happening. It seems that weather has some effect on the changes. In
the summer time the car would drive, when we get to our destination and cut
it off, it will not start when we go to take off. We left the car in the parking lot
and come back in the night time and it starts and we can take it home. In the
morning it doesnt start again. In the winter time we cant turn on the heat for
a long period of time or we fear the gauge will stop working. BTW we can
only feel the heat of AC when turned on high. The settings lower than that
we can not feel much or any air. We have tightened the grounding wire which
got the car going for the fall. In the winter its back to the same stuff as the
winter. Another thing that is alarming, our brake fluid light tends to go off
often: about every 3,000 miles or so. When this happens many of the other
problems follow after. We have even replaced the brake cord because it
broke and our car could not stop suddenly. PLEASE HELP US! I know many
will say that we should give it up and throw in the towel but money is tight
and we weren't planning on buying a car so soon. Even some feed back
from the viewer would be a blessing!