im guessing it would be from a 90 or 91 4.0 ranger
yes, definitely. the explorer harness has all of the headlights and stuff run through the engine harness, and a lot of other stuff run through the dash.
start with a 2nd gen ranger harness, start on the passenger side floor, unhook the computer and push the harness through the firewall. grab the inertia switch while youre there, get the fuse block and relays on the passenger side fender, the whole harness all the way across the engine bay and core support, all the way to the big round plug on the drivers side firewall. there's some headlight stuff that runs through the big round plug, youll figure out you can strip that out since youll still have the original light harness in the truck.
radiators... you dont need anything special. my 2.9 radiator has cooled my 4.0 for 3 or 4 years now with no trouble. as long as you have a manual trans, the radiator is the same. if you have an auto, youll want the radiator for an auto 4.0 explorer. they are thicker, but otherwise the same dimensions.
when i did this swap, i used a 4.0 from a 93 mazda navajo. i already had the harness for it but i realized it would be well worth the time saved to invest in a ranger harness. i also used the computer from a 91-92 explorer (x2x2) because they are supposedly programed a little more gutsy down low since the explorers have more weight over the back end. kept my original dash and gauges, only thing i havent figured out is my tach. it works, but reads 1.5x what it should.