87 wiring

[hatzie]

There are several harnesses and parts of harnesses that terminate at the firewall in the engine bay.
By 1987 they were complex enough that the one-size-fits-all harnesses weren't really useful. IMHO

Forward lamps and horn.
Engine sensors and ignition.
Alternator.
Heat/AC.
Main power.
Fuel and fuel level.
NL2 dual tanks if equipped.
Transmission TCC and or kickdown.
Rear lamps.
Wipers.
If I'm repairing a rear lamp harness I add a 12ga Blue Trailer brake and 12ga Orange trailer battery wires to the four lamp wires and I upgrade the earlier molded connector to the square 2x2 four position Weatherpak that GM used on the 1984 and newer trucks.

How bad is the original harness? Wire jacket cracks and such?

I usually separate the sub-harness in need of TLC from the chassis and lay it out on a sheet of plywood on sawhorses
Once it's laid out I replace the damaged wire or damaged terminal with a new wire and terminals. Do this one at a time with the same or adjusted length of new SXL wire & brand new Delphi (Aptiv Automotive now) terminals.
I replace the wire covering with new pieces where needed. The tape is harness tape not electrical tape. Black split poly loom is easily available from NAPA in shorter lengths along with 100' and 250' spools. I try to be fairly careful of the wiring clips and poly loom Tees and such as I don't have a decent supplier for small quantities, less than 5,000 or 10,000 order size, of these.
I try to re-use the original Delphi connector shells wherever possible.

There are several Delphi wiring booklets that I scanned in the manuals post.