Don’t know about the DVD unit in the caprice but have you checked your fuse box(es) to see whether there is a specific DVD/entertainment unit fuse that is blown (or any fuse that is blown) as any electronics needs power to fire up
Other than that, you’d need to start tracing/checking wires/wiring
Place a DVD in the head unit and flip the screens down in the rear seats. There are a few options to have the DVDs play while the car is in motion to keep the head unit from playing music so the people in the back can use the headphones too.