The answer to most of your questions can be found in the Audio section of this forum. But here is a quick listing,
1991 to 1994 Ford Explorer Wiring/Voltage to Radio
Constant 12V+ Green/Yellow
Switched 12V+ Yellow/Black
Ground Black
Illumination Blue/Red
Dimmer n/a
Amplifier Remote Blue
Antenna Right Front
Front Speakers 5" x 7" Doors
Left Front (+) Orange/Green
Left Front (-) Blue/White
Right Front (+) White/Green
Right Front (-) Green/Orange
Rear Speakers 5" x 7" Rear Doors
Left Rear (+) Pink/Green
Left Rear (-) Brown/Yellow
Right Rear (+) Orange/Red
Right Rear (-) Black/White
Read all the audio stuff. Sooner or later (probably sooner) you will need it all.
I just bought a Boss 638UA In-Dash CD/MP3 Receiver with Front Panel AUX Input & USB
for my 1992 Eddie Bauer Explorer with the JBL sound system. So I'll be writing about my success (I hope) or lack of it in the Audio section.