Swaps done but fuel pump wont kick on

Ok so basically I hacked into the wiring at the point where wires to the rear of the truck are still with the wires for the fuel tank. I wish I knew which red/black was for the pump, the thicker one or the thinner one. I don't want to cut into them to check voltage unless I have too, but definitely thanks for the color breakout, that will help a lot. I am thinking the thinner ones are all for the lights because the thicker wires were yellow and red/black.

If you had a high pressure fuel injection fuel pump, then I would say for sure the thinner is the tail light. It's probably that way anyway. I'll try to track down a harness (thought it won't be from a 87 2.6) and compare the tail light lead to the other wires.

Were you missing the back half of the harness?

No I just was trying to figure out which wire to cut and jump into in order to power up the pump to see if it works.

You should have said what you were trying to do.

Did you figure it out yet? If not, you can check the wiring at the fuel pump relay / control unit. On the later injected models, its behind the driver side kick panel. I'm not sure on your 87 and 88 G54 engine trucks. If the wire there is heavy gauge, then you should know.

Here are a couple of nuggets from an 88 manual for the B2600 (should be the same for the 87):

Well mine is 87 carbed, but I ran 12v to that wire and got nothing from the pump. I thought it was dead and I guess this confirms it. Trying to figure out now a good place to mount an external fuel pump.

You can disconnect the fuel line at the fuel filter, stick it in a clear container, 2 litre cola bottle.
Disconnect the starter engage terminal from the starter and cover in electrical tape or effective ground retardant.
Attempt to start the engine, if the fuel pump can not be heard running and/or no fuel accumulates in the cola bottle, the pump is not working (go from there, power at the pump? etc.) or the tank is too low on fuel for the pump to pick any up.