You have a mess on your hands, but I guess you know that.
Plan A: Read this plan over several times before you get started. It will save you much trouble and aggravation.
It is possible to wire up the fuel pump so that it runs, but you will have to add 3 wires to what you already have.
All wire is 105 ○ C temp rated stranded automotive wire. Try to use the same solid wire color scheme to make it easier to troubleshoot.
DO NOT USE CRIMP SPLICES TO ADD THE WIRES All splices are soldered and covered with 1/4" or 5/16" heat shrink tubing to insure maximum performance and minimum problems with moisture and corrosion. The heat shrink tubing is available at auto parts stores, Radio Shack, Home Depot and electronics supply stores like
http://www.parts-express.com/.
You are going to have to cut the existing wires on the relay socket. See the picture on how to make a proper running wire splice. You may have to unwrap part of the harness to have enough room to work. See the drawing below for the how to do it right.
Use some string to lay out the wire path from one relay to the other. That will give you an idea of the routing and how long the wires need to be. The best path for the wiring is the same one that the harness uses to connect to the computer.
Run 1 length 14 gauge wire from the pink/black wire on the under hood fuel pump relay socket to the orange/lt blue wire on the relay socket under the driver’s seat. This is the power feed for the fuel pump
Run 1 length of 18 gauge wire from the red wire on the under hood fuel pump relay socket to the red wire on the inertia switch. The inertia switch is located the driver's side hatch/trunk under the plastic cover over the taillight. This includes the inertia switch in the fuel pump relay coil circuit to turn the fuel pump off in the event of a collision. This is the power for the fuel pump relay coil.
The original red wire to the inertia switch starts somewhere up near the driver's seat or driver's side kick panel. There is a mystery connector that runs wiring back to the rear of the car in the driver's side kick panel or dash. I have never seen it, but it shows up on the drawings. You may decide to dig for it if you are so inclined. Finding it will save time and wire.
Run 1 length of 18 gauge wire from the lt blue/orange wire on the under hood fuel pump relay socket to tan/lt green wire on the relay socket under the driver’s seat. That is the signal from the computer to close the relay contacts and supply power to the fuel pump.
Use some heat shrink tubing or wire loom to cover the wires you run from the under hood fuel pump relay to the fuel pump relay under the seat.
Plan B:
The simplest answer is to get an 89 5.0 Mustang computer wiring harness and it is plug and go.
Plan C:
The next option is to get an 87-88 5.0 Mustang computer wiring harness and add a Mass Air conversion harness to it.
