I removed the rear heater core on my 84 after it developed a leak.  In order
to maintain the capacity to reinstall a new core I adopted this course of
action.  I replaced the leaky metal "T"s with nylon ones available at your
FLAPS.  I bought a new section of hose to connect the fittings that supplied
the rear heater.  Before installing the new hose I inserted into the hose a
marble to block the coolant flow.  Being glass it is not subject to
corrosion.  I made sure the marble was of sufficient size to block the
coolant flow and was too large to pass through the nylon fittings into the
rest of the cooling system.  If I should decide in the future to replace the
rear core all I have to do is remove the connecting hose and reattach the
"T"s to the heater core.  Simple and cost effective.

Rico Dagastino
Munford, TN