It's mainly the interior, the base 4 cyl/police/5.0 LX interior with the phone dial wheels could be either the civilian '90, and no later, or the police version to '93. No police car ever had the Pony wheels, and no civilian '91-'93 pony wheel equipped car ever had the base seats/interior; all had the sport seats and tweed upholstery. '91-'93 civilian 5.0 LXs were only equipped with the Pony wheels, the phone dials those years were police/SSV only.
Also, the kit has no door panel pockets, which were added in '90, so the door panels are not correct for the '90 anyway. So to make it a correct '91-'93, the Pony wheels will have to be added with wider, lower profile tires, the seats and upholstery will have to be changed, and the door panel pockets will have to be added.
-MJS