Keep in mind mine MB is an 83...so take this for what it's worth. But I THINK the defrost flap is controlled by a dual action pod...could be with the defrost in the off position the pod leaks causing bad shifts.
Good news! As I recall that pod is easy to access by removing the drivers side kick panel and it's right there. Just check/replace the vaccum hose connections first...you may get lucky and fix it for a nickle's worth of tubing.
All this vaccum stuff sucks!
