I HAVE POWER TO THE GLOW PLUGS IN THE HEADS BUT NOT THE INTAKE GLOW PLUG. WHAT TRIGGERS THE SINGLE INTAKE GLOW PLUG? THANKS

 

It's actually not a glowplug, at least in the same sense. Just a heater coil. Referred to by many/most here as the AIH(Air Intake Heater)

Here are the requirements for it to activate

Ambient temperature must be below 32°F
Engine oil temperature (EOT) must be below 131°F
Vehicle power must remain between 11.5 and 15.0 volts
Parking brake must be applied
The transmission must be in Park or Neutral
Glow plugs must be off

 

To add to that....

It has zero to do with cold start.

It was added in late 99 to "help" with white smoke during extended idle in cold conditions.

 

If all the conditions are present for it to come on it is the relay in front of the one for the glowplugs (closest to the plastic "Powerstroke" cover.