I have a 20+ year old Rheem RGAA model upright gas furnace.

Recently had to replace the pilot light thermocouple (it's getting to that age).

The problem we have now is that the blower stays on after the heat has been turned off. I think the reason is that there is a blower control mechanism that has a "sticky" switch. I have no idea if this is due to another bum thermocouple, or a broken relay...

I would love some help figuring out how to debug and fix this.

1.) Is it my thermostat that is causing the issue?
2.) Is it something in the "blower electrical box" that is causing the issue?
3.) Is it something in the "heater electrical box" that is causing the issue?
4.) Is the blower controlled by thermocouple or a timing mechanism?

Thanks!