Sounds like a vacuum problem to me.
Do you get any vacuum to the diaphragm/actuator that is supposed to open that flap?
Kent Bergsma at **************.com has a great vacuum troubleshooting manual for DIY'ers. It explains in laymans terms how everything works and steps you thru how to troubleshoot the problem. The manual is only $20 and I feel is well worth the investment when you own a 300D.
84 500SEL EURO 101K ( JUST LIKE MY 1ST WIFE. GLAD TO GET RID OF HER! )
85 300D 310K (sold)
90 350SDL 184K sold
83 300D 118K (sold)
88 300E 153k (sold)
93 400E 105K (sold)