This is a stepside truck and that makes this a bit less painful. No real body work required.
The original US brake light wire runs from the pedal switch to the turn signal switch in the steering column and the turn switch handles power distribution to the two bright taillight filaments.
This is how I would handle this.
You already have the turn signal wires in place. Don't disturb them. Continue to run the turns off the OEM brake/turn wires. They'll be plain old turn wires when you finish.
Remove the wire that runs from the steering column to the brake lamp switch at the column harmonica plug and the brake light switch. Be sure it's not the one from the cruise control brake pedal switch. And. Be sure to either completely extract it from the harmonica plug on the column and the brake pedal switch and clip off the terminals at both ends... Or. Extract from the pedal plug and heat shrink over the end to avoid shorting out the column switch.
Add a new wire to the brake pedal brake lamp switch and run it to the bulkhead plug... There should be one open Delphi Packard 56 slot dead center in the taillight/fuel-tank bulkhead plug section... so adding it in a neat clean manner should be fairly painless. you'll need male and female Packard 56 terminals Female #12033824 & Male #2971962 for the bulkhead and if 87 uses the same pedal plug as 86 & 88 then you'll need one more Packard 56 female for the pedal plug. The 87 wiring diagrams are a bear to find things like the pedal switch but there are important differences from the easier to read 88 and up RV series diagrams...
Add a new terminal and wire to the engine compartment side of the taillight/fuel tank bulkhead plug that's way longer than you need to reach the rear of the truck.
If you're feeling lazy... Use split poly loom to contain the new wire, from the bulkhead tail/fuel tank plug to the rear, with the old wrapped tail harness rather than pulling that harness out and wrapping the new wire to the old harness with harness tape. 87 may use split poly already so just stuff the new wire inside it.
I believe the four wire tail harness on an '87 terminates in a 4 way Square 2x2 Delphi Weatherpack plug on the LH frame rail at the rear. SO...
Extract the terminals from the old 4 way 2x2 Delphi Weatherpack tower and shroud at the rear taillight harness frame disconnect. Then insert those terminals into a 5 way round Delphi Weatherpack #12034342 Tower & #12065158 Shroud. That way you can neatly add the brake lamp wire right there without any dangling wires or extra connectors. And it looks almost OEM.
You can use 4 way 2x2 Weatherpak tower and shrouds at the taillight pigtails and you'll only need to buy one mating pair... you already have the pair that you took off at the LH frame rail.
There are inexpensive but not cheaply made tools for terminal extraction and crimping these seals and terminals.
The 12155975 ratcheting crimp tool will handle 12-20AWG / 2.00-0.80mm² Packard 56 and Weatherpak terminals and you will not wonder if the terminals are going to stay attached. It's usually right around $100 US and is of decent enough quality that it could be resold.
The 12014012 extractor is usually around $5-$10 US and there's nothing that works quite as well IMHO
Parts List: With No Quantities...
Delphi Packard 56 terminals 14-18AWG / 2.00-1.00mm² Female #12033824
Delphi Packard 56 terminals 14-18AWG / 2.00-1.00mm² Male #2971962
Delphi Weatherpack 4 way square 2x2 black connector #12015024 Shroud
Delphi Weatherpak 4 way square 2x2 black connector #12015798 Tower
Delphi Weatherpak 5 way round connector #12034342 Tower
Delphi Weatherpak 5 way round connector #12065158 Shroud
Delphi Weatherpak Green wire seals Part #15324982
Delphi Weatherpak Female Tower terminals for 14-18AWG / 2.00-1.00mm² wire Part #12124580
Delphi Weatherpak Male Shroud terminals for 14-18AWG / 2.00-1.00mm² wire Part #12124582
The US GM Light Truck wiring diagrams, service manuals, and unit repair manuals etc that I scanned are posted by Chris in the tech section on the board. They should show the tail wiring. You may want to look at a couple other years to get an idea of what you are looking at. 1987 is a bad year for wiring diagram readability but you need em because 86 and 88 have important very big differences in several areas.
http://forum.73-87chevytrucks.com/smforum/index.php?topic=25639.msg213706#msg213706
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