'87 5KCSTQ HVAC temperature control motor--bad gearbox

["Evans, Nicholas" <nevans@geology.state.va.us>]

Clearly this is the season when everyone is trying to make their heater
work & I'm no exception......the little gearbox driven by the small DC
motor down behind the glove compartment (that operates the cable that
operates the various flaps that are supposed to render the interior of
the car comfortable whatever the conditions outside) seems to have lost
a tooth on my car.  It skips a notch about midway through its travel,
with the implication that the cable does not extend or retract as far as
it's supposed to.  Now this condition is not as serious as some that
have been reported on the list: I can bias the thing toward heat or
coolness by adjusting the position of the cable sheathing under the hood
(I can set it so I get plenty of heat, but the heat doesn't fully turn
off when I dial down the temperature setting; or I can set it for cool
wherein I don't get full heating when I want it).  But this is silly,
and I'm into fixing things.  Not having taken one of these thing apart
before (I'm only about 3 weeks into my relationship with this car), is
the gearbox serviceable separately from the motor (not likely)?
Assuming it's not, given that this is a GM-derived system, is it
worthwhile to try & go through GM for a replacement motor/gearbox?  Or,
does anyone out there have a good used unit they'd be willing to part
with?

TIA & Regards,

Nick Evans
Mayflower Garage
Barboursville, Virginia

'87 5KCSTQ