I have a 93 grand am with overheating problems. first it would heat up without boiling ove then when stoped it would boil over. I changed the thermostat and now it just over heats. The radiator was changed about 6 months ago. What could be some possible causes of the overheating and what can one do to fix this. ps the heat and defrost dosent work?

the engine is a 3.3 liter v6, and the fan and fan relay are functioning

the engine is a 3.3 liter v6, and the fan and fan relay are functioning
Check out your water pump, if the impeller is not turning you will not have any coolant flow, thus you will not get heat inside the car. I had this happen in a 77 Olds Cutlass the car would over heat because there was no coolant flow, it turned out to be a bad re-manufactured water pump.

Did you purge the air after changing the tstat ?
That could also cause the no heat situation.
The overheating could be as simple as a rad cap that no longer holds pressure - causeing the coolant to boil at a lower temp.

Ill look at all of those possibilities