1997 Pontiac Trans Am Heater

168 to 175 is not normal if your readings are correct. This engine uses a 195 thermostat and the cooling fans don't come on until about 225 so the first thing you need to do is verify the temp using a laser thermometer.
Someone may have incorrectly removed the thermostat at one time.

With the engine at full operating temp, locate the 2 heater hoses where they go into the firewall and feel them to see if they are both hot to the touch. If not, the issue is in the engine. If they are both hot, the heat in that vehicle is controlled by a blend door that regulates heated air flow. It is operated by an electric motor/actuator. The problem can be that the actuator is stripped or inoperative or the door itself could be damaged. This is what needs to be determined by examining the actuator and see if it is responding to heat change commands or not.

Verify the engine temp first. If the engine is actually getting hot, you may have a restricted heater core.

That's an indication that the heater core is plugged up. Someone may have added sealer to the system at one time or left Dexcool in there for too long,

I would remove the 2 heater hoses and use a garden hose to flush the core in both directions and see if that helps.

Also make sure you have sufficient coolant and the core didn't just freeze.