Ironically, i just completely took the rear interior out of both of my mustangs today (swapping over rear panels from '89 to '84 and gutting the rear of the '89) and from what it looks like it's a direct bolt in with the same bolt holes, even the later style '89 body has provisions for some of the extra bolts for the "sail panel" from the older style mustangs. The window will come with a bracket type thing glued to the window with bolts and nuts to fasten it to the body and you should have no problems. Good luck!
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previous name: 84stangLX
'84 Mustang 5.0 LX
my LX
'89 Mustang GT
my GT