The coolant distribution unit is a plastic piece that routes coolant from
the engine to the radiator and rear heater core. For some reason the syncro
uses a different part than regular vanagons (different hose routing maybe?).
Anyway, VW has made a change on this part you now have to order a plug to go
along with it. Here are the relevant part numbers:

251 121 442 coolant distribution unit
357 121 140 plug
025 121 142 clip
N90316802 O ring

Most dealers are unaware of this change--it took Camelback almost a week to
figure this one out. One has to wonder why VW would make a change to this
part unless it was "improved" for use on newer vehicles and they just
superceded the old part. I guess you can use the plug to drain the coolant
but it seems just as easy to disconnect one of the hoses as to remove the plug.

-Mark McCulley
87 syncro Westfalia, The Snow Leopard