If your heater isn't working then it could be one of a few things, these are the most common:
Heater Control Valve (See >here< for a new one, and >this thread< for how to change it)
Fuse (If you heater blows hot and your interior light has stopped working it's probably the fuse!)
Resistor pack (If you find the fan does not work in certain positions then this would be the first thing to test/replace)
Heater Control Panel (If the resistor pack did not resolve the last issue then it may be a panel malfunction)
Heater Control Valve (See >here< for a new one, and >this thread< for how to change it)
Fuse (If you heater blows hot and your interior light has stopped working it's probably the fuse!)
Resistor pack (If you find the fan does not work in certain positions then this would be the first thing to test/replace)
Heater Control Panel (If the resistor pack did not resolve the last issue then it may be a panel malfunction)