rodents??
Check all the vacuum lines to the selector switch on the dash,and see if the vacuum canister dewhickey is closing the lever on the heater box that switches it from heat to defrost,and make sure you have vacuum in the main hose coming from the engine ,with it running of course!-it sounds like you have vacuum,because you say it works on some of the settings....
If all that is working,then it might be the heater box "doors" might be stuck or have something blocking them from closing,like a pencil dropped down the defroster vent or something..I find a lot of crayons,straws,pencils and pens--and mice nests,dead mice,and decapitated mice in trucks and cars heater boxes around here!--especially if the vehicle has sat a lot..
I had one Subaru my friends mother drove come into my garage with the dashboard shaking voilently whenever the heater was on "high" speed..I took the blower motor out,figuring it was junk,or the fan wheel was broken and throwing it out of balance..found 3 heads of mice inside the fan,and several broken blades out of it--and the rest of the mummified mice where blocking the airflow pretty much to all the ducts!..and a ton of the "stuffing" from under the rugs were packed into it!..
I put them in a baggie and showed them to his mom--she was horrified!..and said "no wonder my cars stinks so bad!--I thought it was my wet dog that stunk the car up!"

--that must be good for your health--inhaling dead mouse particles!

--a quick trip to the boneyard for another blower motor,and all was well..oh yeah..the Subaru at the boneyard had a rats nest in it too--
