Honestly...it makes sense. He didn't hack it.
1988 Mustangs are compatible in terms of wiring with only 1987-1989 Mustangs. For a 4-banger, the body harness is the same, but the engine harness is not. So that means he needed to change the engine harness.
The only compatible years of engine harness would be 1987, 1988 and 1989. Now, only 1 year was mass air, 1989 (unless you get an 1988 CA car) so odds are a speed density 1987-1988 engine harness was more readily available since 1989 harness go for BIG $$$ since they are the only factory OEM harness for Mass Air that swaps into an 1986-1988 speed density Mustang.
So, i bet you have an 87-88 engine harness, a speed density computer and an engine that coule come from any 87-93 Mustang.
Again, not necessarily a poor hack job...but the logical thing the original owner may have done.