Instructions
Travel to the bridge on Route 24 where you must defeat the rocket grunt. After you defeat him, the grunt will tell you about a machine part that he hid in the Cerulean Gym.
Go to the gym and head for the corner in the upper left. Here you will see a buoy. Press "A" when you are near it to find the missing part.
Head to Route 9 and then onto Route 10 to get to the power plant.
Talk to the man you find on a ledge after entering the plant. He will restore power by installing the part for you.
Go east on Route 8 until you come to Lavender Town. Head for the radio tower and talk to the man near the staircase. He will reward you with the radio expansion card.