Fiero GT automatic transmission wont take reverse

Change the filter for starts. When you drop the tranny pan, check for metallic particles in the fluid. If it's clean, change the filter and fill with fresh fluid. You might be okay. If you see metal, your tranny is on it's last legs. You'll probably need to have the tranny serviced, or rebuilt. Or you might get lucky, and changing the filter will do the trick for the moment.