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Below and to the right of brake booster. Blue connector has yellowish thick wire/heavy insulation.
Sorry for the fuzzy picture, it looked better on the phone. The blue connector seems to be a male connector with a very thick insulation. I would imagine this is a power wire of some sort. The other two wires seem to be female connectors but they differ on what can connect to them. I dont see any other devices or accessories that these connectors could plug into.
This is for a 1984 Base F-150 with the 4.9L and C4/C5 transmission.
I have no idea about the furthest-left one, but it looks like it hasn't been connected to anything in quite some time, so...
You're right about the yellow wire with the blue connector, it's used with the optional camper package (and maybe the optional trailer-towing package).
Th elight green/yellow on the right is for the optional underhood lamp.