I have an '01 Blazer and had the same problem. There's a good possibility that the problem is what's called the blend door actuator. The good news is they sell them at Auto Zone, and other auto parts stores and they're not very expensive, $30 to $40. The bad news is where they're at. On the "01 Blazer you have to pull the dash to get to it. I'm not sure on a '99. Maybe one of the Tech's on the board can offer more insight on that.
Based on what I've seen there's not much difference between the '99 and the '01 so here's how I verified mine wasn't working. Under the hood on the passenger side of the firewall is the evaporator housing and there's the fan resistor to the right sidie of the middle of it. Mine has a rubber-like shielding I had to cut out to get to the resistor assembly screws. There are lines to show you where to cut. A box cutter does a nice job and if you're careful it will set right back in without gooping it back in. Anyway you cut that out, remove the connector from the resistor assy. And take out the three screws. When you look in there you can see the blend door. Turn the key to the "on" position, turn the A/C control knob to A/C and turn the temp control knob to full cool. The door should cover the coil you can see from the opening. Turn the knob to full heat and you should see it move and uncover the heater core. If it doesn't move, it's a pretty sure bet it's the actuator.
Three weeks before I replaced the blend door actuator I had to replace my heater core so I had previous experience pulling the dash. There's a thread further down in Blazer repairs titled something like "97 Blazer heater core replacement" There was another member needing to do that and I asked the tech about my truck. He provided lots of good info on how to get the dash pulled back and get to the HVAC box, which is where the actuator is at. I have pics in there showing my truck with the dash pulled back and one of the HVAC box that kind of shows the actuator. Be prepared, it's not easy or quick. The smartest thing I did was buy a box of ziplock bags and keep all the screws sorted and marked as to which part they went to. This saved tons of time and confusion. Apparently, this is a fairly common problem based on the availibility of this part. Just take your time and keep track of everything. It's much cheaper if you can fix this yourself.
Tuesday, August 7th, 2007 AT 2:26 PM