Working on a friend's uncle's 89 with the heat only option that's cable driven, so no vacuum/electronic actuators to fail. We're unable to get heat out of the vents unless sitting still for a period & even then it's not really hot, more lukewarm. As soon as it starts moving again it goes back to lukecool. To start with the motor wouldn't warm up, so after 3 thermostats we got a good one & the motor now comes up to temp. Heater core is too hot to touch. The blend door feels like it's closing correctly (stuck a hand in through the blower motor hole & moved the lever). All the moving parts seem to be moving correctly & we get good flow out of all 3 positions, so we think that eliminates obstructions (although we checked as best we could without completely dismantling the crumbling dash. At this point we're stumped, all we can think of at this point is the cable moving the blend door is slightly stretched from 30 years of use & just isn't closing the door all the way. However we've been unable to find a replacement cable, none of the parts stores/sites seem to even know the manual version existed. Anyone got any ideas or a source for the cable?