Open the top of the radio (rectangular cover, 4 captive screws), then remove the shielding (about 10 T10 Torx screws, be careful not to drop the rear ones down the hole in the back of the radio). Locate S901, its on the front right side of board, its silver with a white plastic tab sticking out (near the display about 1 inch up). Sometimes the switch can get dirty so slide it back and forth a few times to clean the internal contacts.