Welcome!
First, please review the email I sent regarding the guidelines, link, and new member post
Now, as for that plug - it looks like C125 which is for the digital clock. Some trucks had a digital clock in those 3 rectangles above the radio.
Some wiring here:
http://www.garysgaragemahal.com/digital-clock1.htmlTracking down the wiring on these trucks can be "fun".
Putting together details like location, color of plug, number of wires in the plug as well as how many the plug can handle, and the color of the wires will usually help us narrow it down pretty quickly.
For reference, if you have the factory brake controller wiring it will come through the firewall in a grommet all by itself located roughly behind the brake pedal.
Scott
'
Camano' 1986 F250 Supercab XLT Lariat 460/C6
'Chanute' 1980 F350 C&C 400/NP 435 - Gin Pole
But there ain't nothin' wrong with the radio