The panels are way different. If you have the 89 or earlier panels because of the way it is designed there is nowhere for the bar to sit on top of. If you can find a picture of the two panels and look at them side by side it will make sense.
i've put them in both an 87 and 90.. I did a write up on the 90 install. Biggest difference is you can't access the bolts as easy in pre 90 as there are no speaker panels and the templates are different for locaton of the holes.