I purchased my 98 2dr. 4x4 blazer a few years ago in Southern California, after owning it a few moths I realized that the heater doesn't work. I had it looked at when I was getting the tranny replaced, and the mechanic said that everything looked in order. Ant sugggestions...

 

So you do not get any heat at all or the air doesnt come out where you want it to? Your blend door is probably not moving or is broken inside the box. I believe this is pretty common for 98s if not more. I just tore into mine one day and discovered a broken flap door that routes the air from heat to defrost. If you tackle it yourself, make sure you have a whole Saturday free.

 

The blower works fine, it just blow cold air not hot. What box are you talking about?

 

In general, there are two 'doors' (ventilation valves of sort) which control the air in your vehicles interior ventilation. The first, mentioned in the post above, directs air to the desired output (floor vents, defrost vents, or dasboard vents) based on the setting you specify. The second directs air either over the A/C heat exchanger (cold airif the A/C is on or plain outside vent air if it is not)or from over the heater core (hot air). This latter 'door' is controlled by the temperature controls. This latter 'door' may be stuck on the cold setting (either a mechanical problem with the 'door' or the switch which controls it). This would cause everything about your heater to function properly, but the air being blown into the car to simply not pass over the heater core, leaving it as cold (roughly) as the outside air the ventilation system is sucking in.

 

Check under the hood to make sure the heater hoses are infact going to the firewall. Seeing as how a heater core is a 6+ hour job alot of ppl bypass it.
If that checks out feel the heater hoses when the motor is hot. If there not hot, fluch you cooling system and replace the thermostat. If they are hot, you problem lays in the dash somewhere.