temperature gauge stays between 100 degrees and about 125 degrees?

Hello,

It sounds like you have a stuck open thermostat.
When a thermostat fails and stays open, the engine will not reach the operating temperature as the coolant will always flow thru the radiator and cool down the engine.
Which is affect the heater performance. Typically you will get little warm air, not enough to warm up the interior.

Replacing the thermostat is fairly an easy process. The thermostat is located on the upper part of the engine inside the housing. If you follow your upper radiator hose it will lead you right to it.

You might find this video helpful:

https://youtu.be/jhXBAtcutQo

https://www.2carpros.com/articles/how-an-engine-thermostat-works
https://www.2carpros.com/articles/replace-thermostat

I have attached the diagrams for your reference.

Please let me know of any questions.

Thank you.