I have a 1990 Mazda 626. What could be the cause of the heater taking so long to heat up?....could it be the thermostat?

 

Have you recently added any antifreeze? If you do not have a good mix of antifreeze and water, it will take the engine longer to heat up. In other words, do not use straight antifreeze. I use the 50/50 mixture that you can now buy. Just my opinion.

 

I had a similar problem, weak heater performance to no hot air when the car was not moving. Turns out that I'm having a major leak in the radiator. Most of the coolant was gone. No coolant - no hot air from the heater.

M