Progress Thread Father Time: High-$ anti-dent defense system

"Driver's side door-well above the kick panel area" - 9D843 - Cruise Control Module
"A box hanging loose above the pedals" - Rear Window Defrost Control Relay - Should have 18C641 on it somewhere.
"These mount on the brace between the door frame and the steering column bolts" - The olive box with ears is the Wiper Interval Governor. The white box should be 6c625 - Low Oil Warning Relay.

For easier response/reference you should number the pics and/or put down any numbers you see on the components.

I really like that overflow tank

That is really nice, where did you procure that from?

Please post the wire colors with the solid color and stripe color along with the model year of your car. They help identify the circuit and its function. Do not depend on a photograph to do the job, since the camera and then the device the photo is viewed on can alter the color presentation. Clean the wires with some solvent before trying to identify them.

The wiring color codes are a clue to the wire function. A list of the colors is helpful along with the pin to color assignment. The connector shells always have a letter or number on the shell for each pin.
Example:
connector pin A= blue/white
connector pin B = red/green

Ford Factory wiring diagrams have a wire color to function table in the first few pages. The table is generic and is for all Ford products, not just Mustangs. For that reason, you need to think about what sort of options and equipment are in your car.
For example: If the table says wire color is for a power seat, you are looking at the wrong wire color/item type. Fox body Mustangs didn't come with power seats, so you know that isn't what you want.

Ford changed the color for some wires in 91, so that's why the year model is important.

Also show an extra picture of the mating face side of the connector so that we can see the connector shape and how many pins are in the connector. That helps identify the circuit function since most of the connector shapes are unique to the circuit they serve.

The plug shapes on the Ford OEM diagrams match the plug shapes on the actual plug. The diagram is divided up by the layout of the car. The first diagram is for the very front of the car, and it works its way to the rear of the car page by page.