98 4 door blazer 4x4 heater problems

I recently baught a 1998 blazer from a family friend in the summer. Not thinking about heat at the time I never checked to see if it worked. Now that winter has arose I turned on the heat and air blows through for each setting, (1,2,3,4) but all cold air. I talked to my personal mech. and he said to start with the thermostat, I replaced it and nothing changed. I also have full rad. fluid and the temp gauge is normal. Can anyone help me out on fixing it. Maybe some other issues that might be wrong? I don't have money at the moment givin high student loan payments and out of a job. Any help is appreciated im good with repair done a lot of car work myself so if I have a site for videos or whatever it will help ?????????