Tim, yes the core inside the plastic housing is the thing that is leaking.
This is very common in Vanagons. I have new rear heater cores that come with
the seal for the heater valve for $70 each plus shipping. Let me know if I
can help you.
Thanks,
Ken Wilford
http://www.vanagain.com
John 3:16
New Phone (856)-327-4936
Shop: (856)-765-1583
Fax: (856)-327-2242