1986 Jeep GranWagoneer Heater control unit
 

I am having a problem with my heater. The unit does not open the
vents. The motor works as does the temperature slide. The problem is
the top slide does not redirect the air for defrost heat etc. It will
switch to air conditioning though. It appears to be vacuum activated
and all hoses and port appear fine. Where is the vacuum source for
this and how do I reconect it?
Thanks in advance.