What changes are you seeing or referring to? There is no difference in model year 1989's as far as engine harness wiring goes. They're all the same. The only major or important change that happened part way through the 1989 model year is the front subframe and engine mounts. (From January 1989 production -- starting with frame MA70-0119327).
The wiring changed for the 1990 model year. No engine wiring changed partway through the year for any A70's that I've seen (with the exception of minor, inconsequential changes that happened in some pre-89's that do not affect interchangeability), just at model year changes. This is the information I have, and I've never had it be wrong yet:
01/86 - 08/86 = 1986.5 Model Year (Frame Numbers: MA70-0000001 to MA70-0028016)
08/86 - 08/87 = 1987 Model Year (Frame Numbers: MA70-0028017 to MA70-0076230)
08/87 - 08/88 = 1988 Model Year (Frame Numbers: MA70-0076231 to MA70-0104940)
08/88 - 08/89 = 1989 Model Year (Frame Numbers: MA70-0104941 to MA70-0136626)
08/89 - 08/90 = 1990 Model Year (Frame Numbers: MA70-0136627 to MA70-0153209)
08/90 - 08/91 = 1991 Model Year (Frame Numbers: MA70-0153210 to MA70-0159891)
08/91 - 12/92 = 1992 Model Year (Frame Numbers: MA70-0159892 to MA70-0162455)
Off-topic: Many sources and catalogs list production ending in 05/1993, including many Toyota sources like the parts catalog, with the exception of actual frame build sheets which don't actually show any cars produced after December 1992, despite catalogs showing the car in production until 05/1993.
Jeff