Basic heater operation:   Engine coolant rises to approx 180 degrees in
engine block.  Thermostat opens allowing heated coolant to flow through
system into radiator and heater core (works like a small radiator).  Blower
motor fan blows air past heater core thus making heat.

Heater function is not rocket science...

I'm assuming that 'doesn't work properly' means not getting enough heat...
is the heater fan working ???
if engine temp is below normal,  there's your problem... check/replace
thermostat.
if engine temp is normal, perhaps the heater core is plugged...
sometimes normal to high engine temp with little heat from the heater can
mean a very low coolant level ( check for leaks ).
or it could be as simple as the cable slipping off the back of the heat
selector lever on the dash...  this happened on my 94 Miata.  easy to
determine since the lever moved back and forth very freely.

Pete