For an '85 GT to/from a notchback 2M4, no, because it is also a notchback. All notchbacks use the same wiring harness for the tail lamps. All fastbacks use the same harness for their tail lamps. You can switch between the same tail light assembly (like a notchback) from car to car without changing the harness, but if you change a notchback to fastback or a fastback to notchback you have to use the harness that came out of that specific car. There are also potential issues for having to do a little bit of wiring to make the stop lights work depending on what year you have. There are also some minor differences if you have say a six cylinder car from 1985 to 1987 as it will have a blower motor in the harness, but most of the items are rather miniscule that can be worked out.
From your original post though it sounds like you are swapping a notchback body piece for fastback body work, so if you do that yes you will have to do a little bit of extra work.
[This message has been edited by Fiero84Freak (edited 07-04-2012).]