Assuming that there is enough coolant you can check the heater valve. Remove the drive side kick panel, just above the acc. padel you will see heater hoses coming inside the cabin , one of them (I think lower one) goes to heater valve which is operated by lever on the dash.Heater valve is disk shaped flow regulator in my 1990. You can check if the hose exiting the valve is getting hot or not.
In my wagon there is also a 5th speaker grill exactly in the center of dash close to the wind shield,opening that grill exposes top of heater core and shows inlet/outlet hoses