1996 Mazda 626 Question: no heat

Most cars have some form of heater control valve operated directly by a cable going from the heater controls on the dash. The heater controls on the dash may operate a heater control valve by transferring vacuum to the heater control valve either closing the valve (restricting coolant flow through the heater core) or opening the valve (allowing coolant to flow through the heater core). Vacuum operated heater control valves are more common on cars with climate control heating/air conditioning systems. The heater valve is usually located in the engine compartment near the firewall. If your Mazda does indeed have a heater valve see if it is fully opening and closing. I don't think it would be possible to incorrectly hook up the heater core hoses. Could debris have clogged up or restricted the heater core? Perhaps with the engine cold disconnect the heater hoses going to the heater core, block the hoses of at the engine side and refill engine cooling system, connect some additional coolant hoses to the heater core and put a garden hose to either heater hose where it would flow to the heater core. Try flushing it one direction and then the other way, monitoring coolant flow. Briefly start the engine and operate the heater controls to max hot and flush water from the garden hose through the heater core. If operating the heater control lever fully on and off allows what appears to be sufficient flow through the heater radiator perhaps it is a "blend door" issue. Operate all the heater controls functions do you hear "things moving behind the dash"? Sorry I can't be more specific I looked at my information system and it did not have much information. There is an air mix actuator near the heater core under the center of the dash that receives power or grounds to activate the blend doors in the heater box. Perhaps check all your fuses.

Funny.. I'm having the same problem. I have a 2004 Mazda rx-8 that I purchased not even 2 years ago. So far, the engine has had to be replaced; which they paid for since it's a "known problem" with rotary engines although I did have to fight them to cover other parts affected and for the rental car for 2 weeks. The engines are covered up to 100,000 miles. Since that time, my oil light comes on a LOT; I add alot more oil than I used to.. and the coolant light; (reserve was empty). I take it to the dealership regularly for maintenance and now.. no heat. Well, sporadically, I get heat. They want over $600 for a heater control valve. I told them no. Did some research and this is also a "known issue". I called back and spoke with the service dept manager and they are now willing to pay for the "$500 part" if I will pay for labor and for them to "program" the valve???? why does that sound fishy? All this AFTER the engine replacement; re-manufactured engine I believe. I also checked pricing on the internet, and it looks like the part is only $30.00 new. Am I being "screwed over"? lol