1999 GMC suburban 4x4 1500 trailor package

 

First of all you need to check the true operating temperature of your engine (without trusting your gauge). I recommend buying one of those infra-red temperature guns that use a laser to aim it. Check the temp at the top of the radiator, the intake manifold and the hoses going into and out off the the firewall. Your 1999 truck should be operating at 175 to 190 F. If it’s lower than that, consider changing the thermostat.

SD

 

Is the engine reaching operating temperature? If it is then it is not the thermostat. Are both heater hoses hot or is one hotter than the other? If one is hotter than the other then you have a clogged heater core. Or it could be the blend door actuator or a stuck blend door.