Why wont my car heat up?

Feel your heater hoses going to the heater core. If one is hot and the other is cold or Luke warm then your heater core is stopped up. If neither is hot then you have a cooling system issue. If both are hot then you have a blend door issue.

If the heater core is stopped up how do i fix that?

You can try removing the hoses going to the core and flushing water thru the core both ways. You will see a lot of crud come out. When water flows freely both ways thru the core hook hoses back up and you should have good heat. I have had good luck un stopping them this way. Or you could just replace it with a new one. But do your home work first. Make sure that the issue you are having is definitely a stopped up core before going to all the trouble to replace it.

Removing /replacing the heater radiator on my 89 GTC was an utter nightmare... Read up on it, do it as a last resort. Try proprietary radiator flushing chemicals first, that might clear a silted up radiator, or yes, try flushing it in situ one way then the other.

i had my cooling system flushed but i still dont have switchover from cool to heat/defrost