I agree with above -if that is okay then try this:
If you relieve your radiator pressure at the radiator cap (engine cold) and clamp off your heater hoses remove them from the heater core ports at the firewall.
You can run water from a garden hose thru one side of the core and verify the core is not clogged internally. It should flow out the open side very easily. Put pressure to either side. Just put the hose end up to the pipe. This way you don’t over pressurize the core. Water pressure from the hose can be like upwards of 80psi and the core is built for 20psi or less.
Also- is the blower motor working?
Is air coming out but no heat- then the motor is working.
My guess is possible mouse nest blocking core. No easy repair for that one.
