1989 Trans Am GTA Won’t Shift into Reverse or Park

You may want to drop the transmission pan and take a look inside while someone operates the shifter. In automatics, there is a detente spring or some call it a detente roller. Basically, as you rotate the shift arm in the transmission, it rides on the detente spring and gives you feedback in the shifter that you’ve gone from one gear to the next. Without it, your shifter would not “click” or stop as you move it from one gear to the next. This is a total shot in the dark but detente springs have been known to break and jam up the shifter. Most times they can be bought separately and aren’t much harder to change than a transmission filter.