heater temperature control for inside a 2000 lesabre

My 2000 Buick LeSabre heater does not heat up the inside of the car. I have been told that it probably is the heater control valve, but I cannot locate it.

Can you tell me where it is and the best way to access it for replacement?

Thanks,

John

Sorry for the untimely reply. Assuming you have a dual climate control system is that both driver and passenger sides? What do the coolant lines at the firewall going in and out of the heater core feel like?