Heater blowing cool air?

My heater became noisy, air flow was limited and cool. Removed blower on Monday. Found it full of leaves. Cleaned it out and reinstalled and instantly had strong Hot air blowing. Temp gauge was at normal operating temp at this time and normal the entire time I've been driving/ Never showed signs of overheating at all. The next day (Tuesday) I still had strong air flow, blower was working and quiet, but only blowing Cool air now. Coolant level is good, up to neck filler tube and proper mix. All heat controls seem to be working. I'm assuming the door flap is opening when I switch from cool to hot, because on Monday I had heat. I did not do anything. Nothing at all except a visual check of coolant level, didn't need to add any. Temperature gauge still shows normal temperature. No overheating signs at all. Top radiator hose gets hot, bottom hose stays cool or slightly warm. Heater core hoses - has lower one hot and upper one a little warm. I'm assuming the water pump is fine. No signs of leaking and truck not overheating. So would you think my Thermostat is stuck open or closed? Maybe clogged radiator? It did sit for 2 years but I've been driving fine since July and again no overheating symptoms. Or does it point to my heater core? Heater core requires dash removal I think, yikes. If it's heater core, I could use a backflush/unclog procedure from you and hope for the best. Or if thermostat a R and R procedure for that would be appreciated. Or maybe radiator clogged possibility? Lastly, some advice from you on tests to do to isolate and confirm my issue. Thanks again so much, Jose