I have changed the heater motor, blower speed controller and thermostat on myb 2002 Grand marquis but it still blows cold air what else could be the problem

No water getting to heater core. Possibly the valve that controls the flow, allowing water (coolant) in to core is inop, either because the valve itself is stuck shut, or linkage to it. I think that car is vacuum operated to open.close that valve, vacuum hose off maybe, or if you have rusty water in radiator from lack of maintenance it will get the rust particles in it and build up and plug it. But that is unlikely from what you describe by changing thermostat, if you did all that then you know better than to allow your coolant to get so diluted as to rust. But bottom line is back to no water in core. If you can see the valve and is reasonably accessible try to open it by hand.

I will check this problem but I am not sure where the valve is located. What is the name of this valve. I had purchased a heater control valve and could not find where to install it. Is this the valve you are talking about. I didn't think my car had one because I could not find.

It had one unless somebody took it out, in which case there would be water flowing in all the time It looks like it's in the back of the engine, in engine compartment, as opposed to in cabin

Ok thank you I will try this and get back to you to let you know if that was it.

Please do, I'd like to know

might look like this, or similar, and is vacuum operated

Ok thank you again. Much appreciated. Will let you know.

Where is the heater control valve location for a 2000 Mercy Grand Marquis 4.6L engine.