What Tiger is referring to is to disconnect the water hose that comes from
the engine where it goes into the servo. Install some type of shut-off valve
at that point. Disconnect the hose that attaches to the servo and goes into
the firewall and connect it to the other side of the shut-off valve. You
just replace the servo with the shut-off, you just by-pass the servo unit.
The water comes from the engine, through the shut-off valve, then into the
heater. When it's cold outside, open the valve to allow water (hopefully
hot) to flow through the heater. When it's hot outside, turn off the valve
to keep water from flowing to the heater. You are now the cabin thermostat.
In the back yard, under the oak.