How does the heat and air work?

Temperature Control ****
Temperature is controlled by cable-operated blend-air door in plenum assembly. With temperature control
**** in "C" (cool) position, all air is directed around heater core. See Fig. 1. With temp erature control
**** in "W" (warm) position, all air is directed through heater core. When temperature control **** is
between cool and warm positions, air is directed both through and around heater core.
You have either a vacuum leak, or the control panel is defective.
FUNCTION CONTROL SWITCH
1. Remove control panel. See CONTROL PANEL under REMOVAL & INSTALLATION. With
function control switch connector removed from switch, use self-powered test light or ohmmeter
to test switch.
2. To test clutch circuit, connect self-powered test light between Brown /White wire (Light
Blue /Pink wire on "F" Series) terminal and Purple wire terminal. When function control **** is
turned to MAXA/C, NORM A/C, MIX or DEF position, continuity should be present. Continuity
should not be present when function control switch is moved to any other position. Replace
function control switch if it does not test correctly.
3. To test blower motor circuit, connect self-powered test light between MD6Brown/Yellow wire
terminal and Brown /Orange wire terminal. Continuity should be present when function control
**** is moved to any position except OFF.

To get the right diagrams I need an engine size and these are for 5.0 and 5.8 with manual A/C

Usually on one this old start by checking the vacuum reservoir for leaks.