Not sure but the '85 should probably have 3 "slide" levers under the center vent. Use these to shut off the air flow. I have an '84 so I think the dash set up is similar. Also with mine the center vent will be a problem if the lever becomes detached from the flap. Look carefully using a flashlight if you need to through the "gills"of the center vent. Move the slide lever back and forth to see if the flap does indeed operate properly.
If the lever has become detached(through road vibration and time) remove the glove box to ease in reattaching the rod from the lever to the flap. I was only able to remove the center vent gills by pushing them into the dash and out through the glove box opening.
As a precaution, too, be sure to thouroughly clean the space under the brake booster. Over time debris will collect here and the heat given off by the blower motor resistor unit will set off a fire. Believe me...I know. Fortunately for me my car was idling in my driveway with the hood up when my fire started.
Just a thought, check the condition(cold) of the crossover exhaust pipe you see at the back of the engine as you look down on the driverside. Probably reaching here but if the pipe has a hole in it toward the firewall....hope not though.