Look for a purple wire that has a funky looking terminal connector on the end.
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It will originate at the oil pressure
switch.
Thats the one with two connections, not the bell shaped one that has only a single stab (that one is a
sender and provides a variable resistance that drives your dash oil pressure gauge.
The one that powers the choke heater is a simple on/off switch that closes as soon as engine oil pressure reaches 10 psi (basically immediately). That 12 volts heats up the coil inside the choke heater and make it open as quickly as possible.
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Don't be mislead by the 3 wires shown in the pictures, 2 of them are spliced together within the plug body. I am sick of typing - you'll figure it out.
Be aware that, on these trucks, if you see a choke light on your dash while the engine is running it can mean that you have lost oil pressure (or at least droppede below 10 psi)