why does my car cut off when I cut on my heater

Wow, that's a new one. There is a short in the system that is going to need an auto electrician, unless I am missing something and somebody comes along that knows more that I do and explains it. The only other thing I can think of is if you put it on 'defrost' the A/C kicks on and your compressor is locked up, that could stall the car but you should hear a squeal first when the belt tries to turn the compressor

Agree,with fordnut,this could be caused by a weak alternator or battery,as fordnut said,the ac run,s with the heating system when on defrost,and may be pulling engine down and making it cut off,one more thing come,s to mind,the TPS,may be bad and not making up the difference of the pull down on the engine.