1997 Safari/Astro No Heat Issue - Control Valve? Clogged Core? Blend Door? - Let's Figure This Out

It's winter and my heater doesn't work. I've had to make multiple early morning drives and it was tough. With no heat I also had no window defroster.

I'm not a mechanic so my troubleshooting method was probably not the most direct, but in the end I found the problem and now have heat.