It is left of the windshield wiper, over the brake booster, almost behind the electronic cruise control. Where do all these wires disappear to? Does the cruise control work? Is there anything electrical not functioning? It is a shame that only paper service manuals where printed as GM moved into the electronic world in the late 90's.
I would remove the cruise control box to put eyes on where the wires go, like inside via a grommet? Also, if you have a DMM, set to ground, turn on the key, engine off, look to see if you have a reading. If not, try B+ dc on the DMM next and re-try. Check to other wire also and use a buddy to turn on other electrical items like the cruise control switch. I would guess, someone added this aftermarket block to provide B+ to something they had added or did not work as just added a place to get power.
GM used the same color wires for the same circuits to this day, with a designated circuit number that goes back to the 70's best I can tell. Of course, anyone could grab wire of any color they have on hand when fixing something.
Did some work at the dealer, on the used car manager's collectors car, a 1953 Pontiac Chief-Train w/ L-Head 6. It still had the factory "Chief Pontiac" illuminated hood amber glass / chrome ornament. They still use the same front marker lamp circuit BRN as found in a 68 C10. With water pump off, I also replaced the oblong brass coolant "Deflection" tube inside the inline 6 banger. It runs the length of the engine, passenger side with slots to direct coolant between the cylinders. It locked into place at the front using a bell shaped end, pinched onto the block machined recess with the water pumps gasket. I guess casting engine blocks have come a long ways.