Don't specifically know about the Multipla but there should be a drain hole
and hose at each of the front and rear corners of the sunroof tray.  Front
hoses will most probably run down the windscreen pillars into the A-post
area of the car or into the area beneath the windscreen scuttle/engine
compartment.  Rea hoses again most probably down the rear roof section into
that area above the wheel arches an out of the outer floor pan area between
rear wheel arch and back bumper.

Open sun roof and with bright torch, or at night, try and inspect sunroof
tray front/rear corners.  Should be clear of leaves and other debris.
Park on slight slope (front up, then rear up) and using a house plant
watering can pour about a small glass of water into one side of the open
roof area.  e.g. with nose up then rear lhs & rear rhs in turn.  Inspect
under car for emerging water.  It is highly unlikely all tow/four holes are
blocked so one should get a clue as to where water is supposed to come out.

Failing that you may have to drop the headlining to see the underside of the
tray corners.  You should then see the hoses and be able to see where they
go.

Nick /////