Problem with the heating system

I am having a problem with my heating system that no one seems to know the answer to. On the dash there is an AC button to push in or out to use the AC. When I turn on the heat, specifically the defrost, the light on the AC button comes on immediately, which I believe is normal, however, after about 15 minutes the light will start blinking. The air doesn't get nearly as hot as it used to either. Does anyone have any ideas for me?

Thanks

Yeah the light is soupost to come on because when you run your defoggers it turns on the ac with it! witch i think is gay. So i took my ac fuse out so it would blow out hot air. I know the cold air defogs the windows better but when its cold i dont want more cold air in the car! About the light blinking i dont know about that.

About the heater not being hot as normal. Im not sure but in my car even when i had the heater on as hot as it could get it would blow out cold air. It turned out i had a coolent leak. So basicly i dont know and i just wasted some of your time.

hey i am having the same problem but only my heat will come on for 3 minutes then go to cold air...which is really annoying so if you get any good answers please contact me...