why do my heater controls gets only hot and off

Most likely cause is the heater blower motor resistor pack is blown. It's in the engine compartment at the firewall passenger side, 4 wires attached to it black, orange/black, yellow/red and light green/white. For proper tests, there is a company that independent auto repair shops get their information from, this same information is available inexpensively for people that work on their own cars.. The information is year make and model specific, covering repair procedures, torque specifications, fluid capacities and specifications, service bulletins, component locations, wiring diagrams ect.... Alldata is very easy to navigate http://bit.ly/AllData_Repair_Manuals_Online

There is a resistor bank in your heater box to vary the fan speed that is faulty. OFF is off and HIGH doesn't run through the resistor bank,runs direct from switch. follow the wires to the heater usually pass. side below the glove box behind the cover of the heater. You will have to take off USUALLY, I know some Chryslers that are accessable (vans)but not sure about Grand Marquis. Are you a contorsionist(sp)? If not good luck.One note ,it shouldn't affect the heat setting just the fan speed unless it's electronicly controlled by a push button or LCD touch screen then they might work in unison.Then it could be a BCM (body control module) but I doubt it.If you aren't sure call the dealer and ask about it what can they tell you? the answer or go fly.Just say thank you !!!