The electrochromatic mirror needs the special headlight switch, with the plug that goes to the mirror to switch it to night mode when the headlight switch is turned on.
That wiring goes to that plug, and that plastic piece is to cover the wire that runs from the mirror, through the headliner along the windshield, down the A-pillar, and into the dash and to the headlight switch.
The special headlight switches can be very hard to find, even aftermarket, and often very expensive. If you get lucky, you can sometimes find local parts stores that have them in stock and just never sold one over the years. Same with a local Ford dealer, although the price might still be pretty crazy for a factory one.
If your 94 is a limited, it should have the special switch and wiring installed, but if it somehow didn't come with the electrochromatic mirror from the factory, it might not have the switch or wiring either.