not really familar with this car but all are the same ......coolant sytem is full with no leaks >?
Check heater hoses going into heater core and make sure both are hot
If both are hot you have a problem with either heater control cable going to heat and cold air flap ...if its electrical you have a heater control motor bad or a heater control module ....
it can be a blown headgasket too if you have bubbles in coolant recovery jug ......
