What was found
PILOT REPORTED THAT EN ROUTE TO DVL GOT A GPWS C/O & FLICKERING IN TRANSIT LIGHTS WITH 3 GREEN DOWN LOCK INDICATED. FOLLOWED CHECKLIST, HAD NO CHANGE, WAS REROUTED BACK TO MSP, DID NORM EXTENSION, CONF WITH ATC THAT GEAR WAS DOWN, STILL GOT C/O, BUT NO IN TRANSIT LIGHT, DID MANUAL EXTENSION, NO CHANGE, LANDED, DECLARED EMERGENCY. REPLACED THE DOWN LOCK SWITCH, OPERATIONS CHECKED GOOD.