You're thinking about the EEC connector. That's mounted on the firewall near the master cylinder.
I'm sure Encho knew that...But I also think he may be onto something...Considering that, the injectors are the ONLY electrical things within reach of that connector...Maybe some sort of secondary diagnostic tool...
I think that is for the canister purge valve. The valve is located in the vacuum line between the throttle body and the carbon canister and will sit near that plug on top of the lower intake manifold runners.
As stated above, that's for the optional Flux Capacitor. There were only 3 trucks that left the factory with this option installed. You happen to be the proud owner of one of those 3.
You'll have to build a new Flux Capacitor, since Doc Brown is stuck in a different, alternative time period and is unable to help.
But today is your lucky day!! I located a DiY Flux Capacitor instructional web page. You'll have to upgrade the nuclear device located above the spare tire mount to accept garbage to make it work...but I'm sure you have the intelligence to do it. DIY Flux Capacitor
If you are able to make it work, go back to 5/7/1993 and force me to leave with you and talk some sense into me...I don't want to make the mistake of marrying that psycho woman.
That should be the plug for the fuel rail cooling fan to prevent vapor locking on the 4.9L engine.
I forgot about this. Neither my 93 or 95 300s had this. IIRC, on the newer models, they got rid of the fan and raised the fuel pressure to prevent vapor lock. I'm thinking this means that he is also going to need the newer fuel pump for the 92 engine to run properly in his 86 truck.
There's not a fuel rail cooling fan on a '92. I'm reasonably certain that connector fits the canister purge valve - only reason I'm not 100% is I haven't had a look under the hood of my dad's truck in a while (also a '92 w/ the 300).