The red line points out the vacuum like type hose under the dash.

 

There are 2 vacuum lines inside the cabin - they are for the heater controls. The vent selection is vacuum controlled. A line comes in to the controls, and a line runs out to the heater diverter valve. Not all cars had the heater diverter valve. This valve is under the hood, has 3 heater hoses to it and a vacuum line - when set to WARM, the diverter valve opens and allows warm coolant to run through the heater core, when set to COOL it closes in an attempt to keep the cabin temps cooler by not letting the hot coolant into the heater core.

Both of the vacuum lines will enter the firewall just at the bottom driver side corner of the heater box, in the bundle of wires that control the heater blower motor.

 

If if have cruise control. It might be for the cruise control vacuum brake switch. Mounted next to brake light switch.

 

Thats what I was thinking since it seems to be headed in that direction.

 

didn't think of that - don't think my 1992 has any vacuum CC lines, think it's all electronic - I've never seen any vacuum lines at the brake pedal.

 

What was the verdict. Heat or CC