My car's interior heat slowly started to cool down.

The thermostat you can find it by following the lower radiator hose back to the engine the housing is held in by two 10mm bolts. The heater hoses going into the heater core can be found under the intake manifold towards the middle of the firewall. Both hoses should get really hot. Let me know what you find.

A thermostat isnt going to take the heater out a restricted heater core would though. Let me know how those heater hoses feel.

The hoses you have to reach from the engine compartment. If you remove the air intake tube you should be able to reach the drivers side hose from above to see if its hot. If you cant reach the right hose to see if its hot go on the right over below the power steering pump. You will see a metal hose that goes under the upper engine mount. That comes off the heater core see if that hose is really hot. Both hoses should be really hot when the engine is at normal operating temp so becarefull not to burn yourself.

If one hose is really hot and the other one is not then you have restricted heater core. You would replace it from the inside but I would try flushing it to clear it first.