Scott,
What you're describing is called the "coolant distributor".  And I can say from
experience, if it were to blow-out on the road, you need to be towed.  It's
cheap enough - $40 from Camelback VW in Phoenix, including UPS next day air.
Glad to hear your mechanic had one off a '91, but as this thing is plastic, I
wouldn't put a used one in when a new one is relatively inexpensive.  Make sure
you mechanic looks over the hoses that connect to the coolant distributor while
he's changing it out.  Good time to replace any that are questionable or look
suspect.

Alan Bosch
& Phred ('88 Wolfsburg) - New coolant distributor in Dec. '99...
Snowy Rochester, NY