HEAT WORKS ONLY IN DEFROST

Sounds to me that you might have a leak in the line. Those heaters are all Vacume actuated.

What year is your Envoy ? Probably high speed relay .

Mode door actuator . A electric motor that controls a door that directs air flow .

Mode Door 1.
a.This door regulates air flow to either the dash vents or defrost/floor. If you have air flowing only through the vents with no defrost or floor, this door is failing. From our experience, this door is usually the first domino to fail. Not sure why, just is.
<!--[if !supportLists]-->4.Mode Door 2.
a.When mode door 1 is set to divert air away from the vents and into the second stage of the system, door #2 chooses whether air goes to the defrost vents to the windshield or out through the floor vents in the passenger and driver's side footwells. Failure is indicated by an inability to choose between defrost and floor.
b.This door turns out to be the most difficult to replace. In order to replace the door, the dash panel has to be removed. It's still better than the dealer fix because you don't have to evacuate the AC system, drain the radiator, and remove the plenum box. But it is more work than the other fixes. When this door breaks, it falls in the down position, blocking the floor vents and diverting all air through the defrost vents. From a safety standpoint, this is good since you'd most likely rather have a clear windshield than warm feet. We suspect that a large percentage of customers will just leave this door unrepaired. You will have heat through the dash vents and to the windshield, just nothing to the floor. It's inconvenient, but you'll have to make the determination if warm feet are worth the extra work to fix the door. An extra pair of socks is easier.

If you move the dial on the dash and nothing happens, its usually a sign of a blend door problem. The system is designed to default to the defrost position for safety.
Some blend doors are vac operated, some are by small electric motors.
You would need to check the power source for the doors including the control switch.

Sounds like a wiring problem with the harness behind the dash.
You have one or more blend doors that open and close to direct air flow thru the vents. If there is a problem, the default position is defrost for safety.
A mechanic would check the blend doors for voltage and operation.

Near the gas pedal is a linkage that diverts the air from defrost to front to low by your feet. Sometimes this linkage comes un-done. This could be fixed by holding it in place with a wire-tie. Let me know what you find.

This is the KIA tech site. Free to register and use. It lists the repair tech manuals, Sevice Bulletins, How-to's, and troubleshooting guides the factory trained repair guys use:
http://www.kiatechinfo.com/index.asp

radio has 4 holes in front cover 2cloths hangers work well when cut to fit in holesinsert about 3 inches or until you feel springs release gently tug on hangers assembly comes out of dash
save time check heater relay underhood fuse panel

You may need to replace the mode selector switch.