The heater valve has the same problems as the valve in the 240.  I wrote
this last week about a 240:

The valve is located just to the right of the gas pedal, behind the
plastic panel.  First make sure the cable is adjusted properly, so the
valve actually clicks closed in the cool position.  With the plastic
panel off, move the lever and see what is operating, and in what
direction.  Loosen the screw pinching the cable, and see if the lever
will move more in the 'cool' direction.  If that's working OK and you
still have heat, you can tighten the screw (sealed with paint) on the
end of the valve about a quarter turn try to make the shutoff more
positive.

I think the valve on your car is mounted on the firewall, with the
heater hose connections under the hood, and the cable connection under
the dash.  I'm sure you can find it by tracing the cable from the temp
adjust wheel.