Mode door

There isn't a single mode door. There are a number of door that create mode selection and they are all internal to the HVAC housing. The actuator is on the outside and may be accessible.
You really should make your responses to your original question to keep them together.

Sorry I didn't think to reply to my original question from my email. Im just trying to help out a co-worker single mom. So with my limited knoledge im understanding that mode doors direct the flow of air to certain vents, you said its in the hvac housing. I dont have a manual to this car what would I look for to distinguis the housing unit from other components on the car? Im assuming its in the engine compartment close to the fire wall. Im sure once I locate it I will be able to understand how it functions and trouble shoot the problem. Sorry for not having any knoledge of this car. Thanks for any help you can offer.

No, not in the engine compartment. It's buried under the dash and most of the dash would have to be assembled to access the doors themselves. The actuator is usually on the driver's side of the housing, under the dash.

Thanks so the bottom line is that the dash will need to be dissasembled. Sorry for being such a laymen. Do you know of a web site that would offer a repair manual for this car? Thanks so much for your help. My wife ran off with my wallet when she returns I will be glad to make a donation, as I have another question pendening on my 84 benz. You have been very helpfull. Thank you again.

Not only would the dash needs to be removed. The evaporator would be required to be removed as well, meaning you are dealing with A/C freon recovery and recharging of system.

Our database do have information for removing the heater unit, inside which the various doors are located but you should try checking under dash at the linkages on passenger side.