The defroster does not work but the heater works great.

Easy fix you will need some vaccine line ..the blend door works off vaccume the vac line runs off the intake and runs into fire wall find the broken line and fix with the vac line ....let there be heat!!

Vacuum diaphram that works the defrost has gotten a hole in it, or the vacuum hose going to it has come off. Find this diaphram and inspect it! Example of what it looks like is shown.

Great question dustin but the A/C is suppose to come on in defrost or mix mode- this is floor & defrost. Does not seem to make sense does it. One of the things the a/c does is remove the moisture from the air. You need warm DRY air to remove the moisture from the windows. An automotive H.V.A.C. ( Heating,Ventilation,Air conditioning) first forces air thru the a/c evaporator core then thru the heater core and out the selected vent. If you only want a/c a blend door will by-pass the heater core which is controlled by a hot/cold knob or digital temp cotrol on the dash. Now back to the cold air problem- assuming the basics are all ok- Coolant is full, Engine gets up to proper operating temp-around 195 degrees- upper radiator should be hot. Both heater hoses should be hot. If thats all OK the blend door might not be working. It is controlled by an electric motor called a Blend Door Actuator which are prone to fail. I have replaced many on ford vehicles. Hope this helps

Usually a sign of a faulty blend door under the dash.
The blend doors direct air thru the heater core and the desired vents.

make him do it again... ?

For this types of problem,i suggest you to check this help links:----Car heater blowing warm, but not hot air?http://schematicsdiagram.blogspot.com/2011/12/car-heater-blowing-warm-but-not-hot-air.html
---------A/C, heater, defroster fan, not working?http://technoanswers.blogspot.in/2012/02/ac-heater-defroster-fan-not-working.html
--------Heater problem?http://technoanswers.blogspot.in/2012/02/heater-problem-on-isuzu-trooper.html
---------Engine Overheats and heater blowing cold?http://howtobyme.blogspot.com/2011/11/engine-overheats-and-heater-blowing.html
-----------Heat blows cold on drivers side and hot on passenger side?http://schematicsdiagram.blogspot.com/2011/12/heat-blows-cold-on-drivers-side-and-hot.html----------------This will help.Thanks.

there is a actuator that makes a flap flow air out the defrost or the floor they work off vacuum . need to check see if vacuum hoses bad or broken .to get to actuator -the heater box under dash.

the mode switch may be the cause, it work on engine vacuum.
check all the vacuum lines connect to it.
if the vacuum line connects to the defrost actuator came off the you won't have any control

The possible causes you describe are many... Take/tow the vehicle into a Hyundai Service Center (dealership), pay the diagnostic fee. They will tell you EXACTLY what the problem is and what's needed to fix it (parts, labor, tax, time-frame, etc.).

Try checking the connecting hoses to the drivers side defroster vents, i also had the same problem. And i noticed it helped a little if you put in on re-circulate instead of fresh.