Blend door actuator location

Heater not working most of the time at first start in the cold morning the only one the throws warm air is the third from left to right, the other three throw cold air. after driving for a while, i park then when i come back two hours later it work and then stops. today it work after i did a brief stop. i have research online for this problem, it could be the blend door, heater core or something else. also, this was involved in an accident. I purchase it a month ago. could the accident have broken the blend door? also, the fan works, but sometimes i hear a weird noise inside the vents. I would like to replace it myself since is very expensive. or see a how to DIY video on my specific model so I know the locations of blend door if possible. thanks.