I have a 99 tahoe and my heater is always on. I can not turn it off the A/C doesnt turn on. I had recently change the headunit.

Have you replaced the heater core? If not, most likely the reason the previous owner bypassed the heater core was because it leaked. I will be happy to send you photos of how the hoses attach but I will need your email address.

Sorry no idea without a good look

check for a vacuum hose not working the heater flow regulator.(tap)

Usually a sign of a faulty blower resistor. The resistor changes the amperage of the circuit to change the blower speed.
You can look the part up online to get a visual.

https://www.modifiedlife.com/2003-chevrolet-tahoe-radio-wiring-guide/

check your water pump with the belt off and see if it turns freely and doesnt wobble

check the coolant level when it is low there is not enoughflowingthru heater core

The trucks just teasing you.

I Do Believe that the Light Assembly Comes OUT Via TORX Screws and The Lights are On a Curciut Panel that is then Removed to Gain Access to Bulbe

Have you replaced the cab filter ?