Usually cause the power surge protect diode has been destroyed. That's it's job. Sometimes the reverse power problem will travel beyond the diode and burn up a trace on the solder side of the board. Solder a bit of wire to bridge the burnt spot. Rosin core solder. Replace the diode using a suitable size / make of diode. Make a drawing of the location of the silver band on the diode. Called the cathode side. Usually on the board drawn as line, l next to a triangle -l< Scrap electronics should have some you can use. Diode is close to where the power is entering the radio by a largish capicator.