Pop off the intrument cluster cover and find the heater control cable.
Loosen the screw on the cable clamp.  Move the housing so you have a little
more push on the wire.  It should close the valve completely now.  I guess
the cable stretches after 10 to 20 years.  If this doesn't do it, you will
have to look at your hot water valve from underneath (above the spare
tire).  The cable adjustment fixed my '85.
                                                        Jay