If by heater they mean the heater core, (a mini radiator) I only know of 3 ways it can fail.
The most common is that it leaks! Leaking coolant,into the A/C case where it resides will cause the windows to fog up on the inside,
it will usually run out of the A/C case into the passenger compartment until the system is low enough on coolant that there is no hot coolant circulating in the heater core to give you heat.
On the one mirage I saw this happen,the coolant ran down the A/C case on the passenger side and was dripping onto the pass.front floor board.
Yes the dash had to come out to remove the entire A/C case then take the A/C case apart to get the heater core out!
The second way it can fail is that it gets clogged with debris in the cooling system clogging the little passages in the heater core preventing circulation thus rendering it useless to pass on it's heat to the air that is being blown accross it.
This could be checked by perhaps touching the heater hoses and see if it is as hot as the radiator hoses with the engine running at operating temperature! If they are not hot,there is no circulation.
If this was the case,the heater hoses could be removed under the hood and the heater core could be BACKFLUSHED against it's normal circulation to blow the offending particles of debris out where they entered.
The third way of heater core failure could be that the fins are clogged from lint dust,whatever debris blocking AIRFLOW across it's surface thus preventing it from passing on it's heat to the airflow even if there is good hot coolant circulating inside! (this is only likely on older cars with a lot of dirty air blown through the system!)
Without being there diagnosing the car it is hard to tell from the description but like oldmechanic said it could have a temperature actuator issue that is stuck half way between cold and hot or only moves sofar in the hot direction but not all the way or the temp door in the A/C case is stuck and the shaft is stripped where the actuator is trying to turn it,or the actuator itself is stripped where it would be turning the themp door, the list goes on!
Also I'm not sure what has to be removed to access the temp actuator if that turned out to be the issue!
By the way how is your coolant level??? In the reservoir AND in the radiator? That would be the first thing I would check.
Either way it is not really a DIY job so I would just let the dealer fix it unless you are an experienced mechanic!
View my fuel log 2014 Mirage DE 1.2 manual: 66.3 mpg (US) ... 28.2 km/L ... 3.5 L/100 km ... 79.7 mpg (Imp)