Pretested thermostat but still no heat. Could it be there is not enough coolant in the engine?
If the original thermostat wasn’t opening the engine would have been overheating. No cabin heat is rarely caused by a bad thermostat, yet it’s the first thing every DIYer will change. There is usually a valve in one of the hoses that controls the water flow to the heater core. Then there is a door in the box with the heater core that directs the air flow through the A/C evaporator or the heater core. Both of these are controlled by vacuum. Look around under the hood for a broken, cracked or dislodged vacuum hose.
