I am not sure about the 760 with auto climate control but my 1990 745 had a similar problem...

Check the vacuum check valve, (drivers side near brake booster) This valve will only let vacuum flow in the direction toward the engine. It is critical in a turbo because the intake manifold changes from vacuum to pressure under boost. This pressure makes the damper go into a position that seems to turn off the dash maouted discharge. I took the check valve out. blew air through it both directions (with my mouth) banged against something to dislodge any crud, then shot a little WD-40 holding the juice in while I shook it up and down. Put it back in, worked fine. Five minutes total. If the part is bad, I have seen them on the IPD web sight ~$20.