Here's wiring for an '86 4 runner:
1984-86 Toyota 4-Runner Car Stereo Wire Colors
Just get whatever car stereo you want, then use butt connectors to connect the wiring from the stereo to your vehicle's wiring! You have 2 wires per speaker, power, ground, accessory and dimmer control. Plug the black wire with the metal plug on the end (shown in your photo on the very right-most side of the dash) into the antenna jack on the stereo, then mount it all back into your dash.
If you get a standard "din" sized stereo, you'll need a dash kit. If you get a double-din, you will not need the dash kit, but you'll probably need the brackets from toyota to mount it properly. If you feel like tackling this yourself, hit Crutchfield, type in year, make, model, the site will step you through your options.
And that black box is a relay for something, what its for I have no idea. You'll need to trace that wiring back to whatever it goes to. Determine whether you can remove it completely or if its for something necessary. It is definitely not part of Toyota though, so I can only think of it being either for a power antenna, amplifier or light switch that someone maybe had in there before you got it.
Of course, if you just wanna forego all the work and aggravation, just take it to a shop and have them install a deck in there! Installation prices for just a deck in an old toyota are dirt cheap! I used to charge a hundred bucks for installation of a deck and 2 speakers in trucks/4runners back in the the 90's! And, we didn't need no stinkin' dash kits either! We made 'em on-the-fly!!!