I'm 99% sure that the pickups did not have the heater valve, only the Suburbans/Tahoe.
The heater valve has a vacuum line that connects to the solenoid on the firewall. When the a/c control panel temperature dial is set to the full cold detent, the solenoid is powered (from the control panel) and vacuum closes the heater valve.
This, in theory, provides maximum cooling by keeping the coolant from always going through the heater core.
My 1998 K1500 truck does not have it (the heater hoses are routed straight to the heater core connections) but my 1998 Tahoe does have the valve and solenoid.
This isn't my truck, but mine looks like this...no valve, solenoid or its hard vacuum line running across the intake:
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The wiring diagram for 1998 calls for that valve only on the SUV models.
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Heater valve and solenoid on Tahoe:
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To answer your question, the wiring for the solenoid comes from the bulk harness that goes against the firewall, penetrating at the driver side and under the coolant reservoir. From the bulk wiring, a single bundle comes off near the accumulator. It then splits into three pigtails...one goes to the MAF sensor, one to the water/heater valve solenoid and one to the underhood light.
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