The ac in a 2004 hyundai elantra blowing warm air

check for slipping ac drive belt ( smell and noise )

Check your coolant level, low fluid can cause this. Check level with engine fully warm, heater on high. Check your manual to see if you have any air bleeds, these soemtimes look like grease fitting, and are located at higher spots in the cooling system.

Check for a blown fuse. If there is no blown fuse then I woudl replace the heater fan relay.

Neither. You most likely have a leak in the system and lost some refrigerant. Take it to a shop that is licenced to work on A/C

start with a thermostat...

Your water pump is gone or your low on radiator fluid

Your thermostat may not be working properly. Coolant flows thru your heater core where the blower motor blows that heat from the coolant thru your vents. If the coolant is not warm enough wich is a sign that your thermostat may be stuck open or opening too soo, your air may be colder than normal.

are u convinced it is a controller issue as this is rare on that vehicale i am more tempted to say that it is a cable problem. Or a door acuator if it is auto temp control

Well as far as the light goes you have a bad connection, most likely one of the connectors near the bulb or the bulb socket itself.
As far as the AC goes I am sure the problem is the AC compressor cycling switch, this is located in the low pressure AC line, this line gets cold when the AC is on, replace the switch, they are known for sticking and causing a loss of AC cooling.

my 2002 elantra gls sedan started blowing hot air and overheating and it turned out when i took it to the hyundai dealer it was my thermostat they replaced it and i was good to go.it happened at about 40,000 miles hope it works out for you