When you switch to defrost or the air conditioner, the vehicle shuts down completely

What year is your Envoy ? Probably high speed relay .

Problem is usually a stuck or faulty blend door motor under the dash.
The blend doors direct air thru the heater and ac evaporator as well as the vents.

go by liquid coolant and check ur heat and ac fan it might be stuck or broken

well as far as the bog down issue it will bog down a bit in defrost mode because it uses the air conditioner compressor to control humidity does it do the same thing when u use the air conditioner

but if u are putting on the vents it shouldnt use the compressor and if it is then i would that the selector switch is malfunctioning

the heater & defroster baffles are controlled by electic motors that open and close the baffle (doors) to let the air to the heater and defroster vents. the motor must be bad

Check to see if the duct hose to the defroster vents has come loose. If air is blowing out under the dash when you have the control set to defrost, then this is likely.

check for any vaccum leaks, hoses leaking or unhooked, also may have a bad blend door motor. see if all other positions work like floor, vents, fresh air and recycle air. good luck.

THE DEFROSTER DOOR WAS FROZEN SHUT. WHICH CAUSED COMPUTER SWITCH TO GO OUT.HAD TO REPLACE THE DEFROSTER BOX(?) AND COMPUTER SWITCH. WORKS FINE NOW.

Well as far as knowing exactly which hose to put where i cant help. Other than tell you to buy a chiltons manual at advance auto. Now there is a box in the passenger side floor under the dash of your taurus. this box is actually where the flipper is that switches from floor, to vents, and of course your defroster. So you can reach up there manually and switch yours to defrost. Just turn your heater on and(blower motor) then look for the black box should be like one foot by 1 foot. There will be a little siver selector switch on the side of this box. Just flip this switch by hand and you will make your hot air go up to your defroster, also while you are under there, look for loose vacuum hoses, this is normally cause by children hitting these hoses with their feet and disconnecting them. So i am sure now that you know where to actually look you can fix this problem. Hope this helps ya