If you are just installing the dash - then you probably won't need the wiring harness. It is attached to the car itself, not the dash, and the controls in the dash just attach to the harness connectors. If you have the 90 instrument cluster, for it to work you have to reroute some of the wires. see this tech on Stangnet
http://www.stangnet.com/tech/cluster87-93.pdf
Good luck
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Basically stock 87 LX 5.0, owned since 3/31/1987. 216,000+ mile daily driver, GT interior, King Cobra clutch, World Class T-5, GT turbine wheels.
Plans: GT hatch & spoiler, rear disk brakes, and a PAINT JOB!