Re: 2003 Hyundai Elantra 4cyl. How do I get to the mode door for the defroster to check if it's moving freely? Defroster is only blowing air out of ?75 of the vents below windshield.

This is the most likely reason

https://www.fixya.com/cars/r29872074-vehicle_cooling_system_no_heat

It could also be seized blend door actuators not directing the hot air. Or the switch unit.

https://www.google.com/search?q=2003+Hyundai+Elantra+heater+does+not+work

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B lend door not working, There is a electric motor the moves the door when u want air in a different vent, UsuaLLY mounted on heater box.Get a manual on your car and find out where it is,

if you have heat and nothing blows on defrost then this is a mode door problem that is not changing from floor to vent to defrost mode.it could be the mode switch or the mode door not working.the mode door is on the air duct under the dash and you can manually move it when you disconnect the control with your hand it should move freely.some of these have a small motor that moves the door.and if you move the switch in different modes or better have someone move the switch while you see were the door is the one that is moving is the mode door.if it won`t move on defrost tap the motor and see if it starts to work.if it does then it will have to be replaced if not it could be a switch problem.

I think you are talking about the compressor clutch pulley.
This is the one that clicks on & off when you turn on the air conditioner and in many cars the defroster.
Gary

Either the "MODE" control cable has come loose from the actuator pivot connection, or the MODE door inside the air box is broken.

Could be the control switch itself. Or possibly the actuator that moves the door from defrost to heater. With the heater on, try switching from defrost to heat and try to hear if you can hear anything in the dash operating.

On most cars vacuum hoses from the engine power diaphragms that open and close the doors that change where the air blows in the heater system. Check for vacuum leaks. sometimes the diaphragms will go bad. It’s hard to say without actually digging into the system. There might even be something blocking the doors from moving that fell into your defrosters. But at least this gives you a few places to check

Check under your hood for a loose or leaking vac line , The A/C system on your jeep uses manifold vac to move the air mix doors and with no vac the air mix door will move into the defrost position. Find your vac leak and that will fix your problem.

Check for vacuum leak going to air inlet door. Should be a white vacuum hose.

Car manufacturers use the defrost mode as a default for safety reasons. Being they want you to be able to defrost the windshield no matter what. So when something goes wrong the system automatically goes to the defrost mode. You most likely don't have the vacuum hose connected under the dash at the climate control assy. Or if this vehicle doesn't use the vacuum hose style you may have not plugged in one of the connectors under the dash or behind the climate control assy. Good luck.