Just to add to that:
  I too had a leak in the hear heater location and it leaked mostly in the off position before I replaced it.  It was the valve.  All you need to replace is the valve and get a new seal ring too.
  It's not a difficult or expensive repair.  Parts about $40.
  Nate

Mike Collum <collum@VERIZON.NET> wrote:
  Hi Pam,

The rear heater valve is only in one side of the line so the heater is
never really shut off. It remains subject to the pressure of the
cooling system. That valve only shuts off the actual flow of coolant
through the core.

If it only leaks when the valve is in a certain position, then I'd first
suspect the valve itself.

Mike

Pam McIntosh wrote:

> I'm wondering why my '91's rear heater leaks coolant only when heater
> is 'off' - yes off, lever to rear, no heat?
>