for Hal Steiner

Use the biggest stick that will fit. Either side will do and will open both vents. On my car (560SL) I inserted the stick about 8" before I felt it touch the flapper.

Remember, this is a temporary fix to obtain a/c through your center vents. If you are going to leave it unfixed for a length of time, paint your stick black so it won't look quite so funky.