Why isn't my heater not working in 99 Chevy Tahoe?

Probably a blend door issue. switches the system air flow from hot to cold or in between. Check out this guide it will help

https://www.2carpros.com/articles/car-heater-not-working

and

https://www.2carpros.com/articles/replace-blend-door-motor

Please let us know what you find. We are interested to see what it is.

In this case, you are looking for a vacuum leak. The vent door actuators are operated by vacuum from the engine. If there is a leak, they won't change. Check under the dash and hood for broken or damaged hoses. With the engine running, you may get lucky and hear a hissing sound from the leak.

Let me know what you find or if you have other quesitons.

Joe

Start where the control is located.

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 they are.

The temperature is controlled by the use of an air mix door that is operated by an electric motor/actuator. This door can be jammed or the actuator can be inoperative or have stripped plastic gears internally.

Replace the thermostat its cheap could be stuck open and check the heater core if its clogged-up and the heater control valve

The function of the thermostat is to control the engine temperature-if it shut closed it will cause the engine to overheat and stuck open will cause the heater to be blowing cold air and also the engine will never get to the normal operating temp. And computer will be throwing fuel at all times when not needed-

Sounds like your actuator isn't working see pic you cold have two or three the temp door actuator will be the one by itself in center of hvac. Her eis a tip to install it.
When replacing temperature door actuator it seems necessary to remove the ashtray / powerpoint assy. Being careful not to break retainer clips w/o breaking them by squeezing them together on the backside while pulling on the front. This opens it up to easy access to the actuator and screws. The screw on the right side of actuator is easily removed with a 1/4'' socket cap, 5.5 mm socket and a wrench.
Make sure to not hook up the electrical until it is installed and centered.

Flush the heater core both ways to clean of debris. Both hoses should be hot when running.

The door is part of the case and is replaced as an assembly. That may be your issue. Try moving the door manually to see if you get heat

Roy

To me it sounds like the thermastat is stuck open or the collant level is low. With it getting cooler when you drive faster what it is saying is that the fluid is circulating through your engine too fast and the engine not having time to heat it. At higher speeds it flows quicker.