It worked on Friday night, but is only emitting cold air this Saturday morning.

I have had the heater core replaced before. Is this a heater core issue again? Or could be a bad connection somewhere under the hood?

 

When you move the heater controls can you hear the vacuum motors actuate?
they make a hissing noise as doors open and close.
I think the valve to actuate the heat is behind the engine on my 740.
If so I'd look for leaking vac line, unless you have water in the footwells.
One way to find it is to listen after shutting the engine down.
What model is your car?