If you're using the '89 computer, you will need a Mass Air conversion kit. It has a mass air sensor, housing and bracket, and harness. There is also a wire or two which need their positions switched on the (I think) connector at the computer. You probably only want to use the '87 computer harness if it was a V8 car, originally. Also, the Mass Air system does not use the vacuum hose to the MAP sensor. Remove the hose, and cap its source. The Manifold Absolute Pressure sensor functions as a Barometric Pressure sensor with the engine vacuum source eliminated. Ideally, you could get the main harness from the '89. You can get the sensor assembly from the aftermarket.
Or, you could get an '87 5.0 Speed Density computer. If that's what you've got, make sure you connect the afore mentioned vacuum line.
The MAP/BP sensor is up around the center of the firewall.
__________________ 1992 Deep Emerald Green Mustang GT Hatch, 5 speed, 2.73s, Mac fenderwell cold air intake, '93 Cobra MAF, Mac 70mm throttle body & spacer, Explorer intakes, Mac unequal headers & offroad pipe, Magnaflow catback, Walbro 110lph fuel pump, Jet adjustable FPR, MSD Blaster coil, Accel 8mm wires, FRPP aluminum quadrant, UPR firewall adjuster, stock cable, Granatelli upper control arms.
Waiting to go on: Granatelli lower control arms, and GT40 Iron heads! | 
problems with 89 5.0 in 87 body 
