The '81 had a old school 2-knod radio. There is no kit. If the dash has a small square with a hole on each side where the radio goes, you need enlarge the area (cut away the knob holes) to fit a modern radios shape.
Remove the cage from the radio (small metal sleeve), center the cage where the radio belongs (2-knob holes inside cage) trace the cage outline with a fine tip marker or sharp pencil, cut the hole out carefully and precisely. Insert the cage into the new space and bend all the tabs on it outwards to secure it. Slide radio into cage after wiring it to lock in place.
If there is a radio sized rectangular hole there, someone did the grunt work for you. Just remove the cage from the radio, put it in the hole, then bend out the tabs to secure it, wire the radio up, and slip it into the cage.
If you have to enlarge the opening, be careful not to stray off the line. There is very little room for error and any cutting outside the outline of the cage could very well show once the trim plate for the face of the raduo is installed. You will see the bare metal of the cage.
...also, if using something like a razor to cut the dash plastic, make *many* light passes, only cutting a small bit more each pass. Doing this takes less physical effort which translates to less chance of slipping and slicing yourself. It helps keep you on the line too.
