I saw the previous post and it reminded me of my issue.

I get heat from the defrost vents in the summer, less so in the winter.
My footwell vents work mostly, although not as much as i would hope.

The main vents barely have any heat.

I have bled it, i believe that is not the issue. I used to have some waterfall noise in the dash, however that is gone.

When i put it on full blast, i get lots of cold air.
When I put it on auto, there is very limited heat.

The more i think about it, the more it sounds like a heater core, but wouldn't that prevent any material heat?

Start with flushing the core?

 

Start with checking inlet and outlet hoses to heater core with engine hot, both should be hot, if outlet pipe is a lot cooler then your core is partly clogged, flush the heater core, can use a radiator flush just for heater with boiling water, let it sit in heater for a while, then flush with garden hose & then with distilled water, if it isn't completely clogged will clear it somewhat enough to get hot air again, but if really bad, you might need a new core.