Rear Air Conditioning

If your sure you're looking at the right motor, Then the door is broke. Dash and plenum have to come out to fix it. Not a cheap fix.

HVAC Vent Doors under the dash- need to be diagnosed
One is not operating- to switch the air flow
HVAC Vent Door Motor or Vacuum Motor
which ever type you have

Sound is completely normal on all Audi's.
Your A8 uses an Orifice tube to regulate the pressure between the two halves of your air conditioning system. A hissing sound is normal after a shut down as the system equalises. Usually lasts about a minute and is more noticeable on warmer days.
Location on an A8 is on the very right hand side of the bonnet just under the windscreen - that's where the hissing will be coming from.

There is a known issue with these lines on Chrysler vehicles with the rear heat/AC.

A customer satisfaction program exists to make some of these repairs at little or no cost. Contact your local dealer to see if this applies to your vehicle. Have your vans VIN number handy.

Sounds as if you are low on freon.

Check voltage to all three parts: the blower motor, the blower resistor which is directly connected between the motor and the the switch, or also it could be the switch (remember secure you ground the negative side of the voltmeter to either the battery or to the ground car).

Generally the auxiliary climate control system has dual controls for the driver and rear passengers consisting of temperature/air distribution and blower speed; the rear auxiliary climate control assembly is mounted in a separate overhead console positioned on the headliner above the second row seating area.

Also, with the Owners Manual, you can check the relays substuting it in each position in the fuse panel.

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make sure the control knob on the dash is on "RR". This lets the driver control all the vents in the back. The heat only comes out of the floor vents, if you have the knob on the blue dots that represents the air conditioning in which the cool air will come out of the vents up above in the back. There is also an area on the control knob on the dash where you can turn the knob to shut the heat and air conditioning off. If the knob is in this position then you can control everything with the knob up above the seat right behind the driver's seat.

Here is the stereo wires which may help you figure out which are not the ac wires etc. hope this helps with some of the problem.

Constant 12V+ Red/White  Switched 12V+ White/Red  Ground Black  Illumination n/a  Dimmer Orange  Antenna Right Front  Front Speakers 6" x 9" Doors Left Front (+) Light Green/Red  Left Front (-) Light Green/Green  Right Front (+) Light Blue/Red  Right Front (-) Light Blue/Black  Rear Speakers 6 1/2" Rear Door Left Rear (+) Dark Green/White  Left Rear (-) White/Dark Green  Right Rear (+) White/Purple  Right Rear (-) Tan/Dark Green 

yes that is the expansion valve! there is a hissing sound from the restriction in it for a pressure drop thats what makes the a/c cold the hissing is normal

The rear fan fuse is located in the engine compartment on the drivers side to the bottom right of the master cylinder there is a black cover, its right in there it should be a 30 amp fuse, if you flip the fuse cover it will tell you where it is. hope it helps, but if it blown look into the fan or the switch, it blew because of a reason.