@ Twin Turbo I totally agree. Some of the most interesting threads have been under those circumstances.
The thermostat is 100,000 miles old. Sounds like it’s time for a new one.
You have a thermostat with 100,000 miles on it. It could be too old or too non-working. That is the usual suspect when there is insufficient heat, especially if your heater is warmer during warmer months and colder when the weather gets cold.
I’m sure it may be different now but all my old Fords tended to run cold.
Sure change the thermostat but I don’t think that’s the problem. No disrespect but it just seems if it was a simple as a thermostat, they would have found it in two years. We’ll probably never know though.
The OP mentioned in his opening post that he thermostat was replaced. That doesn’t rule it out, but I’m leaning to the heater control valve. I didn’t know until I did one repair on my 2000 Ford Explorer that it had one hidden under a lot of other accessories down near the water pump. Mine is vacuum-actuated, not solenoid lke the video above.