heater blows cold air
What you describe in your 4 words I can deduce there is no water/coolant getting in to your heater core, either because of linkage to valve in heater hose or valve its self
I have had 2 Gran Marquis in my life and both blow barely warm air. My experience: After many years of just barely enough warm air in the Jersey winters, I was in Midas for an oil change and they recommended -in an attempt to get any kind of repair- a cooling system flush. I did it and that was the answer....blows deliciously warm now. Good luck......
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