There are quite a few different harnesses for the A70 USA/Canada Supras. While I am not aware (as it has been a long time since I've had the dash apart), you'll need to find out what harnesses you are plugging into what, and then go from there to find any differences.
The following list is only related to engine harnesses:
87-88 are the only harnesses for the MK3 that are 100% interchangeable.
89 is not interchangeable with any other year (early-style body connectors, later-style ECU connectors).
90 is the first year that has the newer style body connectors and ECU connectors. (last year with the older round O2 connector).
91-92 are very similar to each other, but even they are not identical (different knock sensor connectors, etc.).
90-92 are quite similar, but you'll still need to re-pin a couple body connectors, etc.
If you want a plug & play engine harness, you need to get one from the exact year you have, unless it's a pre-89.
EDIT: Sorry I couldn't help more with the question at hand.
Jeff