The thermal sample input grill is on the top of the dash on the '85 300D, so redirecting the vents would have no effect.
BTW. Have you checked the air sample hose behind the glove box? Remove the glove box and there is a hose that runs from the temp sensor at the center top of the dash to the right. This was a foam hose originally and deteriorates. You may see no trace that there was ever a hose there. If it is gone, go to Home Depot and get a length of foam pipe insulation. Cut to length and slip it over the two connections.
If the hose is missing, replacing it will most likely make your problems go away.