2000 mercury grand marquis air conditioner and heat seem to be running at the same time and defroster will not work.

You have a heater door problem in the dashboard sounds like the AC/Heat door is NOT closing al the way AND the Floor/Defrost Door is not switching either. Sometimes 'things' will fall down the defroster duct and screw up the doors. Disassembly required to clear them. If you are sure there is nothing down there then you may have a broken vacuum line in the engine compartment that feeds the dashboard that is broken. Check against the firewall in the engine compartment on the passenger side - there will be white / red / green plastic lines. There will be one going directly to the dashboard around the heater hose area ( you may also find a hose broken off a mushroom shaped thing at the firewall - It shuts the water going to the heater core) Everything is control by vacuum going to your dashboard control. If you here hissing at the dashboard control - the control its self is bad - Good Luck