Why is no heat in car?

Get someone that knows what they are doing to run the tests. A thermostat is not too bad to replace but the heater core will be. But you must find out where the coolant is going first, and if its a headgasket then that will also be expensive.

If coolant loss was via the heater core, you'd smell coolant inside the car. A thermostat might cause the engine to overheat, but otherwise wouldn't cause coolant loss. If you detect coolant loss from the reservoir, it could be leaking on the ground under the engine/radiator, so LOOK. If you can't see any coolant under the car, you're likely losing it into cylinders due to a blown head gasket or cracked head, so you need to have a mechanic find out where coolant is going before you try to fix anything. None of these things are common to the AVALON, one of the heartiest cars made.

The radiator was leaking, at bottom I had it replaced. Some heat coming from passenger side Was told small motor, under dash that may need replacing But ,entire dash needs to come out.