i just lost my heat in my 1991 cad deville sadan
A "flap on firewall"...uh, I think you misunderstood. A Frirewall is called that for a reason, an engine fire needs to be contained, at least for a while, to the engine compartment. A "flap" I don't understand. If the blower works, and you get no heat, there is a problem with getting hot water to the heater core, assuming when you put in on floor, defrost, dash , whatever and it comes out where you set it. A valve (there have been a lot of similar questions lately) controls the flow of hot water to the heater core, and if that valve or the linkage to it fails, no hot water will get to the core, = no heat. assuming again, your engine is at operating temperature. A "flap on the firewall" is a miscommunication or just plain wrong.
the flap is in the box under the dash alot ar vaccum controlled switches from heat to ac if its not the valve in the line under the hood it could be the linkage or vaccum line to the box under the dash some are switch with a actuator they will go bad u can flip it one way or the other switching from heat to air