Most likely cause of 2004 Yaris no warm air from vents

[B.B._07]

I agree with thebarber that you most likely have air in the system which is preventing you from getting good heat from the heater core. To bleed the air out of the system you need to let the car idle with the radiator cap off until you don't see anymore bubbles coming up. It should take at least 15 or 20 minutes to do as the thermostat needs to open up to let the coolant circulate and let the air out. You'll know the thermostat is open when both the upper and lower radiator hoses feel warm to the touch. Raising the front end of the car or parking at an incline will help get the air out faster. These videos go over it very well:

https://www.youtube.com/watch?v=nThsFGa1vuE

https://www.youtube.com/watch?v=zUpXgAJ1gjU

Also even if you're in a warm climate you don't want to run pure water in the cooling system for the long term as it lacks the anti corrosive additives found in coolant/antifreeze.