No heat problems are common problems faced in many other car models too.
In most of the cases is issue with thermostat.
Thermostat is not a costly part to be replaced.
But if you have already replaced the thermostat,then other possibility is clogged or blocked heater core or faulty heater core.
To confirm this check the heater hoses.There are two heater hoses.One is clamped to the top of radiator and other is clamped to bottom of radiator.
Both the hoses needs to be checked,if one is cold and other is hot,then problem is plugged heater core.
Try flushing heater core and see if that helps,if not then heater core is faulty and needs to be replaced.
This details will help.Thanks.