Chevrolet Repair: 1999 Chevy Blazer - no heat, chevy blazer, air doors

Question
I recently purchased a 99 chevy blazer from a friend, knowing that it has no heat.  They told my that something behind the dash needed to be replaced, but after reading online, I thought it may be associated with the heater core being plugged.  Similar to the forum that I read, the coolant reservoir becomes empty shortly after filling it, but no puddles are noticed on the ground.  Could you please direct me with the steps I need to take to remedy the situation?  Thanks in advance.

Answer
Hi Kevin... check the heater hoses to see if both get hot. if only one does then you can assume that you have a plugged core. if good then check the heat control system. there should be a controller under the dash that moves the heat/ defrost/ cold air doors. sometimes they will sieze up...hope that helps