I think some of it has to do with the gauge clusters. There are several other items to. I have been told the same things about the years not being compatable as you listed and from some people who I know would be correct. (guys like Ray, that build/rebuild these things pretty regular)

I even had one of the newer Ford Racing harnesses and sold it once the problems with using it were pointed out. I cant remember all of them, but it was enough that I did not want to cut up and splice into a new harness, just to be able to use it.

Can Painless make one for a 89?