Test one button at a time for continuity
using a multitester or a continuity tester. Set the multitester to the ohms setting X1 (a continuity tester has no settings and simply lights up to indicate continuity). Press the button on the selector switch for the terminals you are testing. Touch a probe to each terminal. The multitester should change from a reading of infinity to a reading of zero ohms. With the probes still touching the terminals, press a different button. The reading should change back to infinity. Replace the wires and repeat these steps for each button.
If any button fails any test, the selector switch should be replaced.