Why doesn't a 1989 Mercury Grand Marquis heater get hot even after the thermostat was replaced?

Heater could just be air bound and need to be bled It may have a bad heater control valve. If it has one, it will be located on one of the heater hoses by the firewall. Check the valve, and check the vacuum line going to the valve, as it may have rotted, and have come apart where it comes from the firewall, and to the valve. And also, the valve could be frozen in the closed position. If the Heater Control Valve is working-FEEL BOTH hoses-IF one is not HOT then U have a Blockage-Like a blocked heater Core.IF U put a Garden hose on one of the inlets U should have flow (both ways) check 4 a pinched or collapsed hose. If all The Previous items are checked it is possible , If the car is equipped with climate control that this portion of the system is at fault, this was also a common problem with these cars as they aged.