The A9P computer is for mass air and auto.
The 88 mustang came with mass air from
the factory. The 87 mustang came with the speed
density system. Since the 88 mustang came
with mass air then it is unlikely that mass air
sensor harness was spliced in. You could swap
another A9P ecm to see if that is your problem.
These computers are expensive so I wouldn't
go out and buy one until I was sure that was the
problem. You should look into other possibilities
that might be causing your situation first.