If you look at the picture in the first post, the wires pass through the same grommet as the A/C lines, which matches the catalog pic. So it looks to me like that part number is right.

 

Actually, it has nothing to do with emission controls. Some Ford trucks have a similar ambient temp switch that is emission system related but that switch is located on the drivers door jamb. But that is the switch in the picture. My '73 has the same switch mounted on the radiator support just above where the condenser piping passes through the radiator support.
Here is the illustration for the underhood lamp. It doesn't show anything on the rad. support.
Click on the .pdf link to view.

 

From the Ford parts catalog:
D0VY-19618-A
Thermostat Assy., air conditioner evaporator temperature control
Fits 73-74 F100-350 w/ integral A/C
Located in front of condenser core
Motorcraft YH-121
Also, all I can make out of the "fuzzy" engineering number in the picture is D?TA-19618-A?

 

I couldn't make out one single digit of that fuzzy ID number, prolly because of my 66 + year old eyes. If I coulda, wouldn'ta posted that wild mule p!ss inre to the hood compartment lamp.

I looked in the wiring section, found that light, it looked the same, so...

Thanks for the clarification...NumberDummy slinks away....

 

No slinking necessary ND.
Fuzzy is as fuzzy does. Whatever that means.
U still da man.
Hey look it's after five, it's drinkin time.

 

Thanks for the reps turbohunter, much appreciated.

 

No problemo
I really enjoyed this today.
I've been around these trucks for so long I couldn't believe there was a part I didn't know.
Makes it fun eh?
And Bill you are awesome, without your input we wouldn't have found the trail.
Thanks all for a fun day here at work.

 

What your picture showed on the radiator support was NOT a hood light but is a sensor for the A/C. It's meant to be there. All the other stuff they were talking about for the hood light is correct, it just isn't what's in your picture.